Dualism Vs Monism Essay About Myself

Dualism and Mind

Dualists in the philosophy of mind emphasize the radical difference between mind and matter. They all deny that the mind is the same as the brain, and some deny that the mind is wholly a product of the brain. This article explores the various ways that dualists attempt to explain this radical difference between the mental and the physical world. A wide range of arguments for and against the various dualistic options are discussed.

Substance dualists typically argue that the mind and the body are composed of different substances and that the mind is a thinking thing that lacks the usual attributes of physical objects: size, shape, location, solidity, motion, adherence to the laws of physics, and so on. Substance dualists fall into several camps depending upon how they think mind and body are related. Interactionists believe that minds and bodies causally affect one another. Occasionalists and parallelists, generally motivated by a concern to preserve the integrity of physical science, deny this, ultimately attributing all apparent interaction to God. Epiphenomenalists offer a compromise theory, asserting that bodily events can have mental events as effects while denying that the reverse is true, avoiding any threat to the scientific law of conservation of energy at the expense of the common sense notion that we act for reasons.

Property dualists argue that mental states are irreducible attributes of brain states. For the property dualist, mental phenomena are non-physical properties of physical substances. Consciousness is perhaps the most widely recognized example of a non-physical property of physical substances. Still other dualists argue that mental states, dispositions and episodes are brain states, although the states cannot be conceptualized in exactly the same way without loss of meaning.

Dualists commonly argue for the distinction of mind and matter by employing Leibniz's Law of Identity, according to which two things are identical if, and only if, they simultaneously share exactly the same qualities. The dualist then attempts to identify attributes of mind that are lacked by matter (such as privacy or intentionality) or vice versa (such as having a certain temperature or electrical charge). Opponents typically argue that dualism is (a) inconsistent with known laws or truths of science (such as the aforementioned law of thermodynamics), (b) conceptually incoherent (because immaterial minds could not be individuated or because mind-body interaction is not humanly conceivable), or (c) reducible to absurdity (because it leads to solipsism, the epistemological belief that one's self is the only existence that can be verified and known).

Table of Contents

  1. Dualism
  2. Platonic Dualism in the Phaedo
    1. The Argument From Opposites
    2. The Argument From Recollection
    3. The Argument From Affinity
    4. Criticisms of the Platonic Arguments
  3. Descartes' Dualism
    1. The Argument From Indivisibility
    2. Issues Raised by the Indivisibility Argument
    3. The Argument From Indubitability
    4. The Real Distinction Argument
  4. Other Leibniz's Law Arguments for Dualism
    1. Privacy and First Person Authority
    2. Intentionality
    3. Truth and Meaning
    4. Problems with Leibniz's Law Arguments for Dualism
  5. The Free Will and Moral Arguments
  6. Property Dualism
  7. Objections to Dualism Motivated by Scientific Considerations
    1. Arguments from Human Development
    2. The Conservation of Energy Argument
    3. Problems of Interaction
    4. The Correlation and Dependence Arguments
  8. The Problem of Other Minds
  9. Criticisms of the Mind as a Thinking Thing
  10. References and Further Reading

1. Dualism

The most basic form of dualism is substance dualism, which requires that mind and body be composed of two ontologically distinct substances. The term "substance" may be variously understood, but for our initial purposes we may subscribe to the account of a substance, associated with D. M. Armstrong, as what is logically capable of independent existence. (Armstrong, 1968, p. 7). According to the dualist, the mind (or the soul) is comprised of a non-physical substance, while the body is constituted of the physical substance known as matter. According to most substance dualists, mind and body are capable of causally affecting each other. This form of substance dualism is known as interactionism.

Two other forms of substance dualism are occasionalism and parallelism. These theories are largely relics of history. The occasionalist holds that mind and body do not interact. They may seem to when, for example, we hit our thumb with a hammer and a painful and distressing sensation occurs. Occassionalists, like Malebranche, assert that the sensation is not caused by the hammer and nerves, but instead by God. God uses the occasion of environmental happenings to create appropriate experiences.

According to the parallelist, our mental and physical histories are coordinated so that mental events appear to cause physical events (and vice versa) by virtue of their temporal conjunction, but mind and body no more interact than two clocks that are synchronized so that the one chimes when hands of the other point out the new hour. Since this fantastic series of harmonies could not possibly be due to mere coincidence, a religious explanation is advanced. God does not intervene continuously in creation, as the occasionalist holds, but builds into creation a pre-established harmony that largely eliminates the need for future interference.

Another form of dualism is property dualism. Property dualists claim that mental phenomena are non-physical properties of physical phenomena, but not properties of non-physical substances. Some forms of epiphenomenalism fall into this category. According to epiphenomenalism, bodily events or processes can generate mental events or processes, but mental phenomena do not cause bodily events or processes (or, on some accounts, anything at all, including other mental states). (McLaughlin, p. 277) Whether an epiphenomenalist thinks these mental epiphenomena are properties of the body or properties of a non-physical mental medium determines whether the epiphenomenalist is a property or substance dualist.

Still other dualists hold not that mind and body are distinct ontologically, but our mentalistic vocabulary cannot be reduced to a physicalistic vocabulary. In this sort of dualism, mind and body are conceptually distinct, though the phenomena referred to by mentalistic and physicalistic terminology are coextensive.

The following sections first discuss dualism as expounded by two of its primary defenders, Plato and Descartes. This is followed by additional arguments for and against dualism, with special emphasis on substance dualism, the historically most important and influential version of dualism.

2. Platonic Dualism in the Phaedo

The primary source for Plato's views on the metaphysical status of the soul is the Phaedo, set on the final day of Socrates' life before his self-administered execution. Plato (through the mouth of Socrates, his dramatic persona) likens the body to a prison in which the soul is confined. While imprisoned, the mind is compelled to investigate the truth by means of the body and is incapable (or severely hindered) of acquiring knowledge of the highest, eternal, unchanging, and non-perceptible objects of knowledge, the Forms. Forms are universals and represent the essences of sensible particulars. While encumbered by the body, the soul is forced to seek truth via the organs of perception, but this results in an inability to comprehend that which is most real. We perceive equal things, but not Equality itself. We perceive beautiful things but not Beauty itself. To achieve knowledge or insight into the pure essences of things, the soul must itself become pure through the practice of philosophy or, as Plato has Socrates provocatively put it in the dialogue, through practicing dying while still alive. The soul must struggle to disassociate itself from the body as far as possible and turn its attention toward the contemplation of intelligible but invisible things. Though perfect understanding of the Forms is likely to elude us in this life (if only because the needs of the body and its infirmities are a constant distraction), knowledge is available to pure souls before and after death, which is defined as the separation of the soul from the body.

a. The Argument From Opposites

Plato's Phaedo contains several arguments in support of his contention that the soul can exist without the body. According to the first of the Phaedo's arguments, the Argument from Opposites, things that have an opposite come to be from their opposite. For example, if something comes to be taller, it must come to be taller from having been shorter; if something comes to be heavier, it must come to be so by first having been lighter. These processes can go in either direction. That is, things can become taller, but they also can become shorter; things can become sweeter, but also more bitter. In the Phaedo, Socrates notes that we awaken from having been asleep and go to sleep from having been awake. Similarly, since dying comes from living, living must come from dying. Thus, we must come to life again after we die. During the interim between death and rebirth the soul exists apart from the body and has the opportunity to glimpse the Forms unmingled with matter in their pure and undiluted fullness. Death liberates the soul, greatly increasing its apprehension of truth. As such, the philosophical soul is unafraid to die and indeed looks forward to death as to liberation.

b. The Argument From Recollection

A second argument from the Phaedo is the Argument from Recollection. Socrates argues that the soul must exist prior to birth because we can recollect things that could not have been learned in this life. For example, according to Socrates we realize that equal things can appear to be unequal or can be equal in some respects but not others. People can disagree about whether two sticks are equal. They may disagree about if they are equal in length, weight, color, or even whether they are equally "sticks." The Form of Equality—Equality Itself—can never be or appear unequal. According to Socrates, we recognize that the sticks are unequal and that they are striving to be equal but are nevertheless deficient in terms of their equality. Now, if we can notice that the sticks are unequal, we must comprehend what Equality is. Just as I could not recognize that a portrait was a poor likeness of your grandfather unless I already knew what your grandfather looked like, I cannot reccognize that the sticks are unequal by means of the senses, without an understanding of the Form of Equality. We begin to perceive at birth or shortly thereafter. Hence, the soul must have existed prior to birth. It existed before it acquires a body. (A similar argument is developed in Plato's Meno (81a-86b).

c. The Argument From Affinity

A third argument from the Phaedo is the Argument from Affinity. Socrates claims that things that are composite are more liable to be destroyed than things that are simple. The Forms are true unities and therefore least likely ever to be annihilated. Socrates then posits that invisible things such as Forms are not apt to be disintegrated, whereas visible things, which all consist of parts, are susceptible to decay and corruption. Since the body is visible and composite, it is subject to decomposition. The soul, on the other hand, is invisible. The soul also becomes like the Forms if it is steadfastly devoted to their consideration and purifies itself by having no more association with the body than necessary. Since the invisible things are the durable things, the soul, being invisible, must outlast the body. Further, the philosophical soul, that becomes Form-like, is immortal and survives the death of the body.

d. Criticisms of the Platonic Arguments

Some of these arguments are challenged even in the Phaedo itself by Socrates' friends Simmias and Cebes and the general consensus among modern philosophers is that the arguments fail to establish the immortality of the soul and its independence and separability from the body. (Traces of the Affinity argument in a more refined form will be observed in Descartes below). The Argument from Opposites applies only to things that have an opposite and, as Aristotle notes, substances have no contraries. Further, even if life comes from what is itself not alive, it does not follow that the living human comes from the union of a dead (i.e. separated) soul and a body. The principle that everything comes to be from its opposite via a two-directional process cannot hold up to critical scrutiny. Although one becomes older from having been younger, there is no corresponding reverse process leading the older to become younger. If aging is a uni-directional process, perhaps dying is as well. Cats and dogs come to be from cats and dogs, not from the opposites of these (if they have opposites). The Arguments from Recollection and Affinity, on the other hand, presuppose the existence of Forms and are therefore no more secure than the Forms themselves (as Socrates notes in the Phaedo at 76d-e).

We turn now to Descartes' highly influential defense of dualism in the early modern period.

3. Descartes' Dualism

The most famous philosophical work of René Descartes is the Meditations on First Philosophy (1641). In the Sixth Meditation, Descartes calls the mind a thing that thinks and not an extended thing. He defines the body as an extended thing and not a thing that thinks (1980, p. 93). "But what then am I? A thing that thinks. What is that? A thing that doubts, understands, affirms, denies, wills, refuses, and which also imagines and senses." (1980, p. 63). He expands on the notion of extension in the Fifth Meditation saying, "I enumerate the [extended] thing's various parts. I ascribe to these parts certain sizes, shapes, positions, and movements from place to place; to these movements I ascribe various durations" (1980, p. 85). Bodies, but not minds, are describable by predicates denoting entirely quantifiable qualities and hence bodies are fit objects for scientific study.

Having thus supplied us with the meanings of "mind" and "body," Descartes proceeds to state his doctrine: "I am present to my body not merely in the way a seaman is present to his ship, but . . . I am tightly joined and, so to speak, mingled together with it, so much so that I make up one single thing with it" (1980, p. 94). The place where this "joining" was believed by Descartes to be especially true was the pineal gland—the seat of the soul. "Although the soul is joined to the whole body, there is yet in the body a certain part in which it seems to exercise its functions more specifically than in all the others. . . I seem to find evidence that the part of the body in which the soul exercises its functions immediately is. . . solely the innermost part of the brain, namely, a certain very small gland." (1952, p. 294). When we wish to "move the body in any manner, this volition causes the gland to impel the spirits towards the muscles which bring about this effect" (1952, p. 299). Conversely, the body is also able to influence the soul. Light reflected from the body of an animal and entering through our two eyes "form but one image on the gland, which, acting immediately on the soul, causes it to see the shape of the animal." (1952, p. 295-96).

It is clear, then, that Descartes held to a form of interactionism, believing that mental events can sometimes cause bodily events and that bodily events can sometimes cause mental events. (This reading of Descartes-as-interactionist has recently been challenged. See Baker and Morris (1996). Also, Daniel Garber suggests that Descartes is a quasi-occasionalist, permitting minds to act on bodies, but invoking God to explain the actions of inanimate bodies on each other and phenomena where bodies act on minds, such as sensation. See Garber, 2001, ch. 10).

a. The Argument From Indivisibility

Descartes' primary metaphysical justification of the distinction of mind and body is the Argument from Indivisibility. He writes, "there is a great difference between a mind and a body, because the body, by its very nature, is something divisible, whereas the mind is plainly indivisible. . . insofar as I am only a thing that thinks, I cannot distinguish any parts in me. . . . Although the whole mind seems to be united to the whole body, nevertheless, were a foot or an arm or any other bodily part amputated, I know that nothing would be taken away from the mind. . ." (1980, p. 97). Decartes argues that the mind is indivisible because it lacks extension. The body, as an object that takes up space, can always be divided (at least conceptually), whereas the mind is simple and non-spatial. Since the mind and body have different attributes, they must not be the same thing, their "unity" notwithstanding.

This Indivisibility Argument makes use of Leibniz's Law of Identity: two things are the same if, and only if, they have all of the same properties at the same time. More formally, x is identical to y if, and only if, for any property p had by x at time t, y also has p at t, and vice versa. Descartes uses Leibniz's Law to show that the mind and body are not identical because they do not have all of the same properties. An illustration (for present purposes a property can be considered anything that may be predicated of a subject): If the man with the martini is the mayor, it must be possible to predicate all and only the same properties of both "the man" and "the mayor," including occupying (or having bodies that occupy) the same exact spatial location at the same time.

Since divisibility may be predicated of bodies (and all of their parts, such as brains) and may not be predicated of minds, Leibniz's Law suggests that minds cannot be identical to bodies or any of their parts or systems. Although it makes sense to speak of the left or right half of the brain, it makes no sense to speak of half of a desire, several pieces of a headache, part of joy, or two-thirds of a belief. What is true of mental states is held to be true of the mind that has the states as well. In the synopsis of the Meditations, Descartes writes, "we cannot conceive of half a soul, as we can in the case of any body, however small." (1980, p. 52). The mind has many ideas, but they are all ideas of one indivisible mind.

b. Issues Raised by the Indivisibility Argument

John Locke argued that awareness is rendered discontinuous by intervals of sleep, anesthesia, or unconsciousness. (Bk.II, ch.I, sect.10). Is awareness then divisible? Locke suggests that the mind cannot exhibit temporal discontinuity and also have thought as its essence. But even if Descartes was wrong to consider the mind an essentially thinking thing, the concept of mind is not reduced to vacuity if some other, positive characteristic can be found by which to define it. But what might that be? (Without some such means of characterizing the mind it would be defined entirely negatively and we would have no idea what it is).

Against Locke, Dualists can argue in several ways. (1) That the mind has both conscious and unconscious thoughts and that Locke's argument shows only that the mind is not always engaged in conscious reflection, though it may be perpetually busy at the unconscious level. Locke argues that such a maneuver creates grave difficulties for personal identity (Bk.II, Ch.I, sect.11), however, and denies that thoughts can exist unperceived. (2) Dualists can argue that the soul always thinks, but that the memory fails to preserve those thoughts when asleep or under anesthesia. (3) Dualists can argue that the Lockean observation is not relevant to the Argument from Indivisibility because the discontinuity Locke identifies in consciousness is not a spatial discontinuity but a temporal one. The Argument from Indivisibility seeks to show that bodies but not minds are spatially divisible and that argument is not rebutted by pointing out that consciousness is temporally divisible. (Indeed, if minds are temporally divisible and bodies are not, we have an argument for dualism of a different sort).

David Hume, on the other hand, questioned of what the unity of consciousness might consist. The Indivisibility Argument suggests that the mind is a simple unity. Hume finds no reason to grant or assume that the diversity of our experiences (whether visual perception, pain or active thinking and mathematical apprehension) constitute a unity rather than a diversity. For Hume, all introspection reveals is the presence of various impressions and ideas, but does not reveal a subject in which those ideas inhere. Accordingly, if observation is to yield knowledge of the self, the self can consist in nothing but a bundle of perceptions. Even talk of a "bundle" is misleading if that suggests an empirically discoverable internal unity. Thus, Descartes' commitment to a res cogitans or thing which thinks is unfounded and substance dualism is undermined. (For a contrary view on what constitutes the unity of the self, see Madell's view that, "What unites all of my experiences…is simply that they all have the irreducible and unanalyzable property of 'mineness,'" in Nagel, 1986, p. 34, n. 5).

Immanuel Kant replied to Hume that we must suppose or posit the unity of the ego (which he called the "transcendental unity of apperception") as a preliminary to all experience since without such a unity the manifold of sense-data (or "sensibility") could not constitute, for example, the experience of seeing a clock. However, Kant agreed that we must not mistake the unity of apperception for the perception of unity—that is, the perception of a unitary thing or substance. Kant also argued that there is little reason to suppose that the mind or ego cannot be destroyed despite its unity since its powers may gradually attenuate to the point where they simply fade away. The mind need not be separated into non-physical granules to be destroyed since it can suffer a kind of death through loss of its powers. Awareness, perception, memory and the like admit of degrees. If the degree of consciousness decreases to zero, then the mind is effectively annihilated. Even if, as Plato and Descartes agree, the mind is not divisible, it does not follow that it survives (or could survive) separation from the body. Additionally, if the mind is neither physical nor identical to its inessential characteristics (1980, p. 53), it is impossible to distinguish one mind from another. Kant argues that two substances that are otherwise identical can be differentiated only by their spatial locations. If minds are not differentiated by their contents and have no spatial positions to distinguish them, there remains no basis for individuating their identities. (On numerically individuating non-physical substances, see Armstrong, 1968, pp. 27-29. For a general discussion of whether the self is a substance, see Shoemaker, 1963, ch. 2).

c. The Argument From Indubitability

Descartes' other major argument for dualism in the Meditations derives from epistemological considerations. After taking up his celebrated method of doubt, which commits him to reject as false anything that is in the slightest degree uncertain, Descartes finds that the entirety of the physical world is uncertain. Perhaps, after all, it is nothing but an elaborate phantasm wrought by an all-powerful and infinitely clever, but deceitful, demon. Still, he cannot doubt his own existence, since he must exist to doubt. Because he thinks, he is. But he cannot be his body, since that identity is doubtful and possibly altogether false. Therefore, he is a non-bodily "thinking thing," or mind. As Richard Rorty puts it: "If we look in Descartes for a common factor which pains, dreams, memory-images, and veridical and hallucinatory perceptions share with concepts of (and judgments about) God, number, and the ultimate constituents of matter, we find no explicit doctrine. . . . The answer I would give to the question 'What did Descartes find?' is 'Indubitability'" (1979 p. 54). In sum, I cannot doubt the existence of my mind, but I can doubt the existence of my body. Since what I cannot doubt cannot be identical to what I can doubt (by Leibniz's Law), mind and body are not identical and dualism is established.

This argument is also featured in Descartes' Discourse on Method part four: "[S]eeing that I could pretend that I had no body and that there was no world nor any place where I was, but that I could not pretend, on that account, that I did not exist; and that, on the contrary, from the very fact that I thought about doubting the truth of other things, it followed very evidently and very certainly that I existed. . . . From this I knew that I was a substance the whole essence or nature of which was merely to think, and which, in order to exist, needed no place and depended on no material things. Thus this 'I,' that is, the soul through which I am what I am, is entirely distinct from the body. . ." (1980, p. 18).

The Argument from Indubitability has been maligned in the philosophical literature from the very beginning. Most famously, Arnauld comments in the objections originally published with the Meditations that, "Just as a man errs in not believing that the equality of the square on its base to the squares on its sides belongs to the nature of that triangle, which he clearly and distinctly knows to be right angled, so why am I perhaps not in the wrong in thinking that nothing else belongs to my nature which I clearly and distinctly know to be something that thinks, except that fact that I am this thinking being? Perhaps it also belongs to my essence to be something extended." (1912, p. 84). Suppose that I cannot doubt whether a given figure is a triangle, but can doubt whether its interior angles add up to two right angles. It does not follow from this that the number of degrees in triangles may be more or less than 180. This is because the doubt concerning the number of degrees in a triangle is a property of me, not of triangles. Similarly, I may doubt that my body is not a property of my body, believing it to be a property of whatever part of me it is that doubts, and that "whatever" may be something extended.

The dualist can reply in two ways. First, he or she may argue that, while doubting the body is not a property of bodies, being doubtable is a property of bodies. Since bodies have the property of being doubtable, and minds do not, by Leibniz's Law the diversity of the two is established. Second, the dualist may reply that it is always possible to doubt whether the figure before me is a triangle. As such, Arnauld's supposedly parallel argument is not parallel at all. Similar objections are open against other, more recent rebuttals to Descartes' argument. Consider, for example, the following parallel argument from Paul Churchland (1988, p. 32): I cannot doubt that Mohammed Ali was a famous heavyweight boxer but can doubt that Cassius Clay was a famous heavyweight boxer. Following Descartes, it ought to be that Ali is not Clay (though in fact Clay was a famous heavyweight and identical to Ali). By way of reply, surely it is possible for an evil demon to deceive me about whether Mohammed Ali was a famous heavyweight boxer. So, the dualist might insist, the case of mind is unique in its immunity from doubt. It is only with reference to our own mental states that we can be said to know incorrigibly.

d. The Real Distinction Argument

A third argument in the Meditations maintains that the mind and body must really be separate because Descartes can conceive of the one without the other. Since he can clearly and distinctly understand the body without the mind and vice versa, God could really have created them separately. But if the mind and body can exist independently, they must really be independent, for nothing can constitute a part of the essence of a thing that can be absent without the thing itself ceasing to be. If the essence of the mind is incorporeal, so must be the mind itself.

4. Other Leibniz's Law Arguments for Dualism

a. Privacy and First Person Authority

As noted earlier, dualists have argued for their position by employing Leibniz's Law in many ingenious ways. The general strategy is to identify some property or feature indisputably had by mental phenomena but not attributable in any meaningful way to bodily or nervous phenomena, or vice versa. For example, some have suggested that mental states are private in the sense that only those who possess them can know them directly. If I desire an apple, I know that I have this desire "introspectively." Others can know of my desire only by means of my verbal or non-verbal behavior or, conceivably, by inspection of my brain. (The latter assumes a correlation, if not an identity, between nervous and mental states or events). My linguistic, bodily and neural activities are public in the sense that anyone suitably placed can observe them. Since mental states are private to their possessors, but brain states are not, mental states cannot be identical to brain states. (Rey pp. 55-56).

A closely related argument emphasizes that my own mental states are knowable without inference; I know them "immediately." (Harman, 1973, pp. 35-37). Others can know my mental states only by making inferences based on my verbal, non-verbal or neurophysiological activity. You may infer that I believe it will rain from the fact that I am carrying an umbrella, but I do not infer that I believe it will rain from noticing that I am carrying an umbrella. I do not need to infer my mental states because I know them immediately. Since mental states are knowable without inference in the first person case, but are knowable (or at least plausibly assigned) only by inference in the third person case, we have an authority or incorrigibility with reference to our own mental states that no one else could have. Since beliefs about the physical world are always subject to revision (our inferences or theories could be mistaken), mental states are not physical states.

b. Intentionality

Some mental states exhibit intentionality. Intentional mental states include, but are not limited to, intendings, such as plans to buy milk at the store. They are states that are about, of, for, or towards things other than themselves. Desires, beliefs, loves, hates, perceptions and memories are common intentional states. For example, I may have a desire for an apple; I may have love for or towards my neighbor; I may have a belief about republicans or academics; or I may have memories of my grandfather.

The dualist claims that brain states, however, cannot plausibly be ascribed intentionality. How can a pattern of neural firings be of or about or towards anything other than itself? As a purely physical event, an influx of sodium ions through the membrane of a neural cell creating a polarity differential between the inside and outside of the cell wall, and hence an electrical discharge, cannot be of Paris, about my grandfather, or for an apple. [Although Brentano goes further than most contemporary philosophers in regarding all mental phenomena as intentional, he argues that "the reference to something as an object is a distinguishing characteristic of all mental phenomena. No physical phenomena exhibits anything similar." (Brentano, 1874/1973, p. 97, quoted in Rey, 1997, p. 23).] Thus, by Leibniz's Law, if minds are capable of intentional states and bodies are not, minds and bodies must be distinct. (Taylor, pp. 11-12; Rey pp. 57-59).

c. Truth and Meaning

Another attempt to derive dualism by means of Leibniz's Law observes that some mental states, especially beliefs, have truth-values. My belief that it will rain can be either true or false. But, the dualist may urge, as a purely physical event, an electrical or chemical discharge in the brain cannot be true or false. Indeed, it lacks not only truth, but also linguistic meaning. Since mental states such as beliefs possess truth-value and semantics, it seems incoherent to attribute these properties to bodily states. Thus, mental states are not bodily states. Presumably, then, the minds that have these states are also non-physical. (Churchland, 1988, p. 30; Taylor, 1983, p. 12).

d. Problems with Leibniz's Law Arguments for Dualism

Although each of these arguments for dualism may be criticized individually, they are typically thought to share a common flaw: they assume that because some aspect of mental states, such as privacy, intentionality, truth, or meaning cannot be attributed to physical substances, they must be attributable to non-physical substances. But if we do not understand how such states and their properties can be generated by the central nervous system, we are no closer to understanding how they might be produced by minds. (Nagel, 1986, p. 29). The question is not, "How do brains generate mental states that can only be known directly by their possessors?" Rather, the relavent question is "How can any such thing as a substance, of whatever sort, do these things?" The mystery is as great when we posit a mind as the basis of these operations or capacities as when we attribute them to bodies. Dualists cannot explain the mechanisms by which souls generate meaning, truth, intentionality or self-awareness.Thus, dualism creates no explanatory advantage. As such, we should use Ockham's razor to shave off the spiritual substance, because we ought not to multiply entities beyond what is necessary to explain the phenomena. Descartes' prodigious doubt notwithstanding, we have excellent reasons for thinking that bodies exist. If the only reasons for supposing that non-physical minds exist are the phenomena of intentionality, privacy and the like, then dualism unnecessarily complicates the metaphysics of personhood.

On the other hand, dualists commonly argue that it makes no sense to attribute some characteristics of body to mind; that to do so is to commit what Gilbert Ryle called a "category mistake." For example, it makes perfect sense to ask where the hypothalamus is, but not, in ordinary contexts, to ask where my beliefs are. We can ask how much the brain weighs, but not how much the mind weighs. We can ask how many miles per hour my body is moving, but not how many miles per hour my mind is moving. Minds are just not the sorts of things that can have size, shape, weight, location, motion, and the other attributes that Descartes ascribes to extended reality. We literally could not understand someone who informed us that the memories of his last holiday are two inches behind the bridge of his nose or that his perception of the color red is straight back from his left eye. If these claims are correct, then some Leibniz's Law arguments for dualism are not obviously vulnerable to the critique above.

5. The Free Will and Moral Arguments

Another argument for dualism claims that dualism is required for free will. If dualism is false, then presumably materialism, the thesis that humans are entirely physical beings, is true. (We set aside consideration of idealism—the thesis that only minds and ideas exist). If materialism were true, then every motion of bodies should be determined by the laws of physics, which govern the actions and reactions of everything in the universe. But a robust sense of freedom presupposes that we are free, not merely to do as we please, but that we are free to do otherwise than as we do. This, in turn, requires that the cause of our actions not be fixed by natural laws. Since, according to the dualist, the mind is non-physical, there is no need to suppose it bound by the physical laws that govern the body. So, a strong sense of free will is compatible with dualism but incompatible with materialism. Since freedom in just this sense is required for moral appraisal, the dualist can also argue that materialism, but not dualism, is incompatible with ethics. (Taylor, 1983, p. 11; cf. Rey, 1997, pp. 52-53). This, the dualist may claim, creates a strong presumption in favor of their metaphysics.

This argument is sometimes countered by arguing that free will is actually compatible with materialism or that even if the dualistic account of the will is correct, it is irrelevant because no volition on the part of a non-physical substance could alter the course of nature anyway. As Bernard Williams puts it, "Descartes' distinction between two realms, designed to insulate responsible human action from mechanical causation, insulated the world of mechanical causation, that is to say, the whole of the external world, from responsible human action. Man would be free only if there was nothing he could do." (1966, p. 7). Moreover, behaviorist opponents argue that if dualism is true, moral appraisal is meaningless since it is impossible to determine another person's volitions if they are intrinsically private and otherworldly.

6. Property Dualism

Property dualists claim that mental phenomena are non-physical properties of physical phenomena, but not properties of non-physical substances. Property dualists are not committed to the existence of non-physical substances, but are committed to the irreducibility of mental phenomena to physical phenomena.

An argument for property dualism, derived from Thomas Nagel and Saul Kripke, is as follows: We can assert that warmth is identical to mean kinetic molecular energy, despite appearances, by claiming that warmth is how molecular energy is perceived or manifested in consciousness. Minds detect molecular energy by experiencing warmth; warmth "fixes the reference" of heat. ("Heat" is a rigid designator of molecular motion; "the sensation of heat" is a non-rigid designator.) Similarly, color is identical to electromagnetic reflectance efficiencies, inasmuch as color is how electromagnetic wavelengths are processed by human consciousness. In these cases, the appearance can be distinguished from the reality. Heat is molecular motion, though it appears to us as warmth. Other beings, for example, Martians, might well apprehend molecular motion in another fashion. They would grasp the same objective reality, but by correlating it with different experiences. We move toward a more objective understanding of heat when we understand it as molecular energy rather than as warmth. in our case, or as whatever it appears to them to be in theirs. Consciousness itself, however, cannot be reduced to brain activity along analogous lines because we should then need to say that consciousness is how brain activity is perceived in consciousness, leaving consciousness unreduced. Put differently, when it comes to consciousness, the appearance is the reality. Therefore, no reduction is possible. Nagel writes:

Experience . . . does not seem to fit the pattern. The idea of moving from appearance to reality seems to make no sense here. What is the analogue in this case to pursuing a more objective understanding of the same phenomena by abandoning the initial subjective viewpoint toward them in favor of another that is more objective but concerns the same thing? Certainly it appears unlikely that we will get closer to the real nature of human experience by leaving behind the particularity of our human point of view and striving for a description in terms accessible to beings that could not imagine what it was like to be us. (Nagel 1974; reprinted in Block et. al. p. 523).

Consciousness is thus sui generis (of its own kind), and successful reductions elsewhere should give us little confidence when it comes to experience.

Some property dualists, such as Jaegwon Kim, liken "having a mind" to "a property, capacity, or characteristic that humans and some higher animals possess in contrast with things like pencils and rocks. . . . Mentality is a broad and complex property." (Kim, 1996, p. 5). Kim continues: "[Some properties] are physical, like having a certain mass or temperature, being 1 meter long, and being heavier than. Some things—in particular, persons and certain biological organisms—can also instantiate mental properties, like being in pain and liking the taste of avocado." (p. 6). Once we admit the existence of mental properties, we can inquire into the nature of the relationship between mental and physical properties. According to the supervenience thesis, there can be no mental differences without corresponding physical differences. If, for example, I feel a headache, there must be some change not only in my mental state, but also in my body (presumably, in my brain). If Mary is in pain, but Erin is not, then, according to the supervenience thesis, there must be a physical difference between Mary and Erin. For example, Mary's c-fibers are firing and Erin's are not. If this is true, it is possible to argue for a type of property dualism by arguing that some mental states or properties, especially the phenomenal aspects of consciousness, do not "supervene on" physical states or properties in regular, lawlike ways. (Kim, p. 169).

Why deny supervenience? Because it seems entirely conceivable that there could exist a twin Earth where all of the physical properties that characterize the actual world are instantiated and are interrelated as they are here, but where the inhabitants are "zombies" without experience, or where the inhabitants have inverted qualia relative to their true-Earth counterparts. If it is possible to have mental differences without physical differences, then mental properties cannot be identical to or reducible to physical properties. They would exist as facts about the world over and above the purely physical facts. Put differently, it always makes sense to wonder "why we exist and not zombies." (Chalmers, 1996, p. 110). (Kim, 169 and following.; Kripke, 1980, throughout; Chalmers, 1996, throughout, but esp. chs. 3 & 4).

Some have attempted to rebut this "conceivability argument" by noting that the fact that we can ostensibly imagine such a zombie world does not mean that it is possible. Without the actual existence of such a world, the argument that mental properties do not supervene on physical properties fails.

A second rebuttal avers that absent qualia thought experiments (and inverted spectra though experiments) only support property dualism if we can imagine these possibilities obtaining. Perhaps we think we can conceive a zombie world, when we really can't. We may think we can conceive of such a world but attempts to do so do not actually achieve such a conception.

To illustrate, suppose that Goldbach's Conjecture is true. If it is, its truth is necessary. If, then, someone thought that they imagined a proof that the thesis is false, they would be conceiving the falsity of what is in reality a necessary truth. This is implausible. What we should rather say in such a case is that the person was mistaken, and that what they imagined false was not Goldbach's Conjecture after all, or that the "proof" that was imagined was in fact no proof, or that what they were really imagining was something like an excited mathematician shouting, "Eureka! So it's false then!" Perhaps it is likewise when we "conceive" a zombie universe. We may be mistaken about what it is that we are actually "picturing" to ourselves. Against this objection, however, one could argue that there are independent grounds for thinking that the truth-value of Goldbach's theorem is necessary and no independent reasons for thinking that Zombie worlds are impossible; therefore, the dualist deserves the benefit of the doubt.

But perhaps the physicalist can come up with independent reasons for supposing that the dualist has failed to imagine what she claims. The physicalist can point, for example, to successful reductions in other areas of science. On the basis of these cases she can argue the implausibility of supposing that, uniquely, mental phenomena resist reduction to the causal properties of matter. That is, an inductive argument for reduction outweighs a conceivability argument against reduction. And in that case, the dualist must do more than merely insist that she has correctly imagined inverted spectra in isomorphic individuals. (For useful discussions of some of these issues, see Tye 1986 and Horgan 1987.)

7. Objections to Dualism Motivated by Scientific Considerations

The Ockham's Razor argument creates a strong methodological presumption against dualism, suggesting that the mind-body split multiplies entities unnecessarily in much the way that a demon theory of disease complicates the metaphysics of medicine compared to a germ theory. It is often alleged, more broadly, that dualism is unscientific and renders impossible any genuine science of mind or truly empirical psychology.

a. Arguments from Human Development

Those eager to defend the relevance of science to the study of mind, such as Paul Churchland, have argued that dualism is inconsistent with the facts of human evolution and fetal development. (1988, pp. 27-28; see also Lycan, 1996, p. 168). According to this view, we began as wholly physical beings. This is true of the species and the individual human. No one seriously supposes that newly fertilized ova are imbued with minds or that the original cell in the primordial sea was conscious. But from those entirely physical origins, nothing non-physical was later added. We can explain the evolution from the unicellular stage to present complexities by means of random mutations and natural selection in the species case and through the accretion of matter through nutritional intake in the individual case. But if we, as species or individuals, began as wholly physical beings and nothing nonphysical was later added, then we are still wholly physical creatures. Thus, dualism is false. The above arguments are only as strong as our reasons for thinking that we began as wholly material beings and that nothing non-physical was later added. Some people, particularly the religious, will object that macro-evolution of a species is problematic or that God might well have infused the developing fetus with a soul at some point in the developmental process (traditionally at quickening). Most contemporary philosophers of mind put little value in these rejoinders.

b. The Conservation of Energy Argument

Others argue that dualism is scientifically unacceptable because it violates the well-established principle of the conservation of energy. Interactionists argue that mind and matter causally interact. But if the spiritual realm is continually impinging on the universe and effecting changes, the total level of energy in the cosmos must be increasing or at least fluctuating. This is because it takes physical energy to do physical work. If the will alters states of affairs in the world (such as the state of my brain), then mental energy is somehow converted into physical energy. At the point of conversion, one would anticipate a physically inexplicable increase in the energy present within the system. If it also takes material energy to activate the mind, then "physical energy would have to vanish and reappear inside human brains." (Lycan, 1996, 168).

The dualists' basically have three ways of replying. First, they could deny the sacredness of the principle of the conservation of energy. This would be a desperate measure. The principle is too well established and its denial too ad hoc. Second, the dualist might offer that mind does contribute energy to our world, but that this addition is so slight, in relation to our means of detection, as to be negligible. This is really a re-statement of the first reply above, except that here the principle is valid in so far as it is capable of verification. Science can continue as usual, but it would be unreasonable to extend the law beyond our ability to confirm it experimentally. That would be to step from the empirical to the speculative—the very thing that the materialist objects to in dualism. The third option sidesteps the issue by appealing to another, perhaps equally valid, principle of physics. Keith Campbell (1970) writes:

The indeterminacy of quantum laws means that any one of a range of outcomes of atomic events in the brain is equally compatible with known physical laws. And differences on the quantum scale can accumulate into very great differences in overall brain condition. So there is some room for spiritual activity even within the limits set by physical law. There could be, without violation of physical law, a general spiritual constraint upon what occurs inside the head. (p. 54)

Mind could act upon physical processes by "affecting their course but not breaking in upon them" (1970, p. 54). If this is true, the dualist could maintain the conservation principle but deny a fluctuation in energy because the mind serves to "guide" or control neural events by choosing one set of quantum outcomes rather than another. Further, it should be remembered that the conservation of energy is designed around material interaction; it is mute on how mind might interact with matter. After all, a Cartesian rationalist might insist, if God exists we surely wouldn't say that He couldn't do miracles just because that would violate the first law of thermodynamics, would we?

c. Problems of Interaction

The conservation of energy argument points to a more general complaint often made against dualism: that interaction between mental and physical substances would involve a causal impossibility. Since the mind is, on the Cartesian model, immaterial and unextended, it can have no size, shape, location, mass, motion or solidity. How then can minds act on bodies? What sort of mechanism could convey information of the sort bodily movement requires, between ontologically autonomous realms? To suppose that non-physical minds can move bodies is like supposing that imaginary locomotives can pull real boxcars. Put differently, if mind-body interaction is possible, every voluntary action is akin to the paranormal power of telekinesis, or "mind over matter." If minds can, without spatial location, move bodies, why can my mind move immediately only one particular body and no others? Confronting the conundrum of interaction implicit in his theory, Descartes posited the existence of "animal spirits" somewhat subtler than bodies but thicker than minds. Unfortunately, this expedient proved a dead-end, since it is as incomprehensible how the mind could initiate motion in the animal spirits as in matter itself.

These problems involved in mind-body causality are commonly considered decisive refutations of interactionism. However, many interesting questions arise in this area. We want to ask: "How is mind-body interaction possible? Where does the interaction occur? What is the nature of the interface between mind and matter? How are volitions translated into states of affairs? Aren't minds and bodies insufficiently alike for the one to effect changes in the other?"

It is useful to be reminded, however, that to be bewildered by something is not in itself to present an argument against, or even evidence against, the possibility of that thing being a matter of fact. To ask "How is it possible that . . . ?" is merely to raise a topic for discussion. And if the dualist doesn't know or cannot say how minds and bodies interact, what follows about dualism? Nothing much. It only follows that dualists do not know everything about metaphysics. But so what? Psychologists, physicists, sociologists, and economists don't know everything about their respective disciplines. Why should the dualist be any different? In short, dualists can argue that they should not be put on the defensive by the request for clarification about the nature and possibility of interaction or by the criticism that they have no research strategy for producing this clarification.

The objection that minds and bodies cannot interact can be the expression of two different sorts of view. On the one hand, the detractor may insist that it is physically impossible that minds act on bodies. If this means that minds, being non-physical, cannot physically act on bodies, the claim is true but trivial. If it means that mind-body interaction violates the laws of physics (such as the first law of thermodynamics, discussed above), the dualist can reply that minds clearly do act on bodies and so the violation is only apparent and not real. (After all, if we do things for reasons, our beliefs and desires cause some of our actions). If the materialist insists that we are able to act on our beliefs, desires and perceptions only because they are material and not spiritual, the dualist can turn the tables on his naturalistic opponents and ask how matter, regardless of its organization, can produce conscious thoughts, feelings and perceptions. How, the dualist might ask, by adding complexity to the structure of the brain, do we manage to leap beyond the quantitative into the realm of experience? The relationship between consciousness and brain processes leaves the materialist with a causal mystery perhaps as puzzling as that confronting the dualist.

On the other hand, the materialist may argue that it is a conceptual truth that mind and matter cannot interact. This, however, requires that we embrace the rationalist thesis that causes can be known a priori. Many prefer to assert that causation is a matter for empirical investigation. We cannot, however, rule out mental causes based solely on the logic or grammar of the locutions "mind" and "matter." Furthermore, in order to defeat interactionism by an appeal to causal impossibility, one must first refute the Humean equation of causal connection with regularity of sequence and constant conjunction. Otherwise, anything can be the cause of anything else. If volitions are constantly conjoined with bodily movements and regularly precede them, they are Humean causes. In short, if Hume is correct, we cannot refute dualism a priori by asserting that transactions between minds and bodies involve links where, by definition, none can occur.

Some, such as Ducasse (1961, 88; cf. Dicker pp. 217-224), argue that the interaction problem rests on a failure to distinguish between remote and proximate causes. While it makes sense to ask how depressing the accelerator causes the automobile to speed up, it makes no sense to ask how pressing the accelerator pedal causes the pedal to move. We can sensibly ask how to spell a word in sign language, but not how to move a finger. Proximate causes are "basic" and analysis of them is impossible. There is no "how" to basic actions, which are brute facts. Perhaps the mind's influence on the pineal gland is basic and brute.

One final note: epiphenomenalism, like occasionalism and parallelism, is a dualistic theory of mind designed, in part, to avoid the difficulties involved in mental-physical causation (although occasionalism was also offered by Malebranche as an account of seemingly purely physical causation). According to epiphenomenalism, bodies are able to act on minds, but not the reverse. The causes of behavior are wholly physical. As such, we need not worry about how objects without mass or physical force can alter behavior. Nor need we be concerned with violations of the conservation of energy principle since there is little reason to suppose that physical energy is required to do non-physical work. If bodies affect modifications in the mental medium, that need not be thought to involve a siphoning of energy from the world to the psychic realm. On this view, the mind may be likened to the steam from a train engine; the steam does not affect the workings of the engine but is caused by it. Unfortunately, epiphenomenalism avoids the problem of interaction only at the expense of denying the common-sense view that our states of mind have some bearing on our conduct. For many, epiphenomenalism is therefore not a viable theory of mind. (For a defense of the common-sense claim that beliefs and attitudes and reasons cause behavior, see Donald Davidson.)

d. The Correlation and Dependence Arguments

The correlation and dependence argument against dualism begins by noting that there are clear correlations between certain mental events and neural events (say, between pain and a-fiber or c-fiber stimulation). Moreover, as demonstrated in such phenomena as memory loss due to head trauma or wasting disease, the mind and its capacities seem dependent upon neural function. The simplest and best explanation of this dependence and correlation is that mental states and events are neural states and events and that pain just is c-fiber stimulation. (This would be the argument employed by an identity theorist. A functionalist would argue that the best explanation for the dependence and correlation of mental and physical states is that, in humans, mental states are brain states functionally defined).

Descartes himself anticipated an objection like this and argued that dependence does not strongly support identity. He illustrates by means of the following example: a virtuoso violinist cannot manifest his or her ability if given an instrument in deplorable or broken condition. The manifestation of the musician's ability is thus dependent upon being able to use a well-tuned instrument in proper working order. But from the fact that the exhibition of the maestro's skill is impossible without a functioning instrument, it hardly follows that being skilled at playing the violin amounts to no more than possessing such an instrument. Similarly, the interactionist can claim that the mind uses the brain to manifest it's abilities in the public realm. If, like the violin, the brain is in a severely diseased or injurious state, the mind cannot demonstrate its abilities; they of necessity remain private and unrevealed. However, for all we know, the mind still has its full range of abilities, but is hindered in its capacity to express them. As for correlation, interactionism actually predicts that mental events are caused by brain events and vice versa, so the fact that perceptions are correlated with activity in the visual cortex does not support materialism over this form of dualism. Property dualists agree with the materialists that mental phenomena are dependent upon physical phenomena, since the fomer are (non-physical) attributes of the latter. Materialists are aware of these dualist replies and sometimes invoke Ockham's razor and the importance of metaphysical simplicity in arguments to the best explanation. (See Churchland, 1988, p. 28). Other materialist responses will not be considered here.

8. The Problem of Other Minds

The problem of how we can know other minds has been used as follows to refute dualism. If the mind is not publicly observable, the existence of minds other than our own must be inferred from the behavior of the other person or organism. The reliability of this inference is deeply suspect, however, since we only know that certain mental states cause characteristic behavior from our own case. To extrapolate to the population as a whole from the direct inspection of a single example, our own case, is to make the weakest possible inductive generalization. Hence, if dualism is true, we cannot know that other people have minds at all. But common sense tell us that others do have minds. Since common sense can be trust, dualism is false.

This problem of other minds, to which dualism leads so naturally, is often used to support rival theories such as behaviorism, the mind-brain identity theory, or functionalism (though functionalists sometimes claim that their theory is consistent with dualism). Since the mind, construed along Cartesian lines, leads to solipsism (that is, to the epistemological belief that one's self is the only existence that can be verified and known), it is better to operationalize the mind and define mental states behaviorally, functionally, or physiologically. If mental states are just behavioral states, brain states, or functional states, then we can verify that others have mental states on the basis of publicly observable phenomena, thereby avoiding skepticism about other selves.

Materialist theories are far less vulnerable to the problem of other minds than dualist theories, though even here other versions of the problem stubbornly reappear. Deciding to define mental states behaviorally does not mean that mental states are behavioral, and it is controversial whether attempts to reduce mentality to behavioral, brain, or functional states have been successful. Moreover, the "Absent Qualia" argument claims that it is perfectly imaginable and consistent with everything that we know about physiology that, of two functionally or physiologically isomorphic beings, one might be conscious and the other not. Of two outwardly indistinguishable dopplegangers, one might have experience and the other none. Both would exhibit identical neural activity; both would insist that they can see the flowers in the meadow and deny that they are "blind"; both would be able to obey the request to go fetch a red flower; and yet only one would have experience. The other would be like an automaton. Consequently, it is sometimes argued, even a materialist cannot be wholly sure that other existing minds have experience of a qualitative (whence, "qualia") sort. The problem for the materialist then becomes not the problem of other minds, but the problem of other qualia. The latter seems almost as severe an affront to common sense as the former. (For an interesting related discussion, see Churchland on eliminative materialism, 1988, pp. 43-49.)

9. Criticisms of the Mind as a Thinking Thing

We earlier observed that some philosophers, such as Hume, have objected that supposing that the mind is a thinking thing is not warranted since all we apprehend of the self by introspection is a collection of ideas but never the mind that purportedly has these ideas. All we are therefore left with is a stream of impressions and ideas but no persisting, substantial self to constitute personal identity. If there is no substratum of thought, then substance dualism is false. Kant, too, denied that the mind is a substance. Mind is simply the unifying factor that is the logical preliminary to experience.

The idea that the mind is not a thinking thing was revived in the twentieth century by philosophical behaviorists. According to Gilbert Ryle in his seminal 1949 work The Concept of Mind, "when we describe people as exercising qualities of mind, we are not referring to occult episodes of which their overt acts and utterances are effects; we are referring to those overt acts and utterances themselves." (p. 25). Thus, "When a person is described by one or other of the intelligence epithets such as 'shrewd' or 'silly', 'prudent' or 'imprudent', the description imputes to him not the knowledge, or ignorance, of this or that truth, but the ability, or inability, to do certain sorts of things." (p. 27). For the behaviorist, we say that the clown is clever because he can fall down deliberately yet make it look like an accident We say the student is bright because she can tell us the correct answer to complex, involved equations. Mental events reduce to bodily events or statements about the body. As Ludwig Wittgenstein notes in his Blue Book:

It is misleading then to talk of thinking as of a "mental activity." We may say that thinking is essentially the activity of operating with signs. This activity is performed by the hand, when we think by writing; by the mouth and larynx, when we think by speaking; and if we think by imagining signs or pictures, I can give you no agent that thinks. If then you say that in such cases the mind thinks, I would only draw your attention to the fact that you are using a metaphor. (1958, p. 6)

John Wisdom (1934) explains: "'I believe monkeys detest jaguars' means 'This body is in a state which is liable to result in the group of reactions which is associated with confident utterance of 'Monkeys detest jaguars,' namely keeping 'favorite' monkeys from jaguars and in general acting as if monkeys detested jaguars.'" (p. 56-7).

Philosophical behaviorism as developed by followers of Wittgenstein was supported in part by the Private Language Argument. Anthony Kenny (1963) explains:

Any word purporting to be the name of something observable only by introspection (i.e. a mental event)... would have to acquire its meaning by a purely private and uncheckable performance . . . If the names of the emotions acquire their meaning for each of us by a ceremony from which everyone else is excluded, then none of us can have any idea what anyone else means by the word. Nor can anyone know what he means by the word himself; for to know the meaning of a word is to know how to use it rightly; and where there can be no check on how a man uses a word there is no room to talk of "right" and "wrong" use (p. 13).

Mentalistic terms do not have meaning by virtue of referring to occult phenomena, but by virtue of referring to something public in a certain way. To understand the meaning of words like "mind," "idea," "thought," "love," "fear," "belief," "dream," and so forth, we must attend to how these words are actually learned in the first place. When we do this, the behaviorist is confident that the mind will be demystified.

Although philosophical behaviorism has fallen out of fashion, its recommendations to attend to the importance of the body and language in attempting to understand the mind have remained enduring contributions. Although dualism faces serious challenges, we have seen that many of these difficulties can be identified in its philosophical rivals in slightly different forms.

10. References and Further Reading

  • Aristotle, Categories.
  • Armstrong, D. M.: A Materialist Theory of the Mind (Routledge and Kegan Paul, London 1968) Chapter Two.
  • Baker, Gordon and Morris, Katherine J. Descartes' Dualism (Routledge, London 1996).
  • Block, Ned, Owen Flanagan, and Gueven Guezeldere eds. The Nature of Consciousness: Philosophical Debates (MIT Press, Cambridge 1997).
  • Brentano, Franz: Psychology from an Empirical Standpoint trans. A. Rancurello, D. Terrell, and L. McAlister (Routledge and Kegan Paul, London 1874/1973).
  • Broad, C. D. Mind and Its Place in Nature (Routledge and Kegan Paul, London 1962).
  • Campbell, Keith: Body and Mind (Anchor Books, Doubleday & Co., Garden City NJ 1970).
  • Chalmers, David J.: The Conscious Mind: In Search of a Fundamental Theory (Oxford University Press, Oxford 1996).
  • Churchland, Paul: Matter and Consciousness, Revised Edition (MIT Press, Cambridge MA 1988).
  • Davidson, Donald: "Actions, Reasons and Causes" The Journal of Philosophy 60 (1963) pp. 685- 700, reprinted in The Philosophy of Action, Alan White, ed. (Oxford University Press, Oxford 1973).
  • Descartes, Rene: Discourse on Method and Meditations on First Philosophy, Donald A. Cress trans. (Hackett Publishing Co., Indianapolis 1980).
  • Descartes, Rene: Descartes' Philosophical Writings, selected and translated by Norman Kemp Smith (Macmillan, London 1952).
  • Descartes, Rene: The Philosophical Works of Descartes, vol.2, Elizabeth S. Haldane, trans. (Cambridge University Press, 1912).
  • Dicker, Georges: Descartes: An Analytical and Historical Introduction (Oxford University Press, Oxford 1993).
  • Ducasse, C. J.: "In Defense of Dualism" in Dimensions of Mind, Sydney Hook, ed. (Macmillan, NY 1961).
  • Garber, Daniel: Descartes Embodied: Reading Cartesian Philosophy through Cartesian Science (Cambridge University Press, Cambridge 2001).
  • Harman, Gilbert: Thought (Princeton University Press, Princeton 1973).
  • Hart, William D. "Dualism" in A Companion to the Philosophy of Mind, Samuel Guttenplan, ed. (Basil Blackwell, Oxford 1994) pp. 265-269.
  • Horgan, Terence: "Supervenient Qualia" Philosophical Review 96 (1987) pp. 491-520.
  • Hume, David: An Enquiry Concerning Human Understanding (Hackett Publishing, Indianapolis 1977).
  • Joad, C. E. M.: How Our Minds Work (Philosophical Library, 1947).
  • Kant, Immanuel: Critique of Pure Reason, Norman Kemp Smith, trans. (Macmillan, London 1963).
  • Kenny, Anthony: Action, Emotion and the Will (Routledge & Kegan Paul, London 1963).
  • Kim, Jaegwon: Philosophy of Mind (Westview Press, Boulder 1996).
  • Kripke, Saul: Naming and Necessity (Harvard University Press, Cambridge 1980).
  • Locke, John: Essay Concerning Human Understanding vol. 1, collated and annotated by Alexander Fraser (Dover Publications, NY 1959).
  • Lycan, William: "Philosophy of Mind" in The Blackwell companion to Philosophy, Nicholas Bunnin and E. P. Tsui-James eds. (Blackwell Publishers, Oxford 1996).
  • Madell, G. The Identity of the Self (Edinburgh University Press, 1983).
  • Malcolm, Norman: "Knowledge of Other Minds" in The Philosophy of Mind, V. C. Chappell, ed. (Prentice-Hall , Englewood Cliffs 1962).
  • McLaughlin, Brian P.: "Epiphenomenalism" in A Companion to the Philosophy of Mind, Samuel Guttenplan, ed. (Blackwell, Oxford 1994).
  • Mill, J. S. : An Examination of Sir William Hamilton's Philosophy, 6th ed. (Longman's, London 1889).
  • Nagel, Thomas: "Brain Bisection and the Unity of Consciousness" in Jonathan Glover, ed. The Philosophy of Mind (Oxford University Press, Oxford 1976).
  • Nagel, Thomas: The View From Nowhere (Oxford University Press, Oxford 1986).
  • Nagel, Thomas: "What Is It Like to Be a Bat?" Philosophical Review 83 (1974) 435-450.
  • Plato: Meno.
  • Plato: Phaedo.
  • Rey, Georges: Contemporary Philosophy of Mind (Blackwell Publishers, Cambridge 1997).
  • Rorty, Richard: Philosophy and the Mirror of Nature (Princeton University Press, Princeton NJ 1979).
  • Rorty, Richard: "Mind-Body Identity, Privacy and Categories" in The Mind/Brain Identity Theory, C. V. Borst ed. (Macmillan, London 1970).
  • Ryle, Gilbert: The Concept of Mind, (University Paperbacks, Barnes & Noble, NY 1949).
  • Shoemaker, Sydney: Self-Knowledge and Self-Identity (Cornell University Press, Ithaca 1963).
  • Taylor, Richard: Metaphysics, 3rd edition (Prentice Hall, Englewood Cliffs 1983).
  • Tye, Michael: "The Subjective Qualities of Experience" Mind 95 (Jan. 1986) pp. 1-17.
  • Williams, Bernard: "Freedom and the Will" in Freedom and the Will, D. F. Pears, ed. (Macmillan, London 1966).
  • Wisdom, John: Problems of Mind and Matter, (Cambridge University Press, 1934).
  • Wittgenstein, Ludwig: The Blue and Brown Books (Harper & Row, NY 1958).

Author Information

Scott Calef
Email: swcalef@owu.edu
Ohio Wesleyan University
U. S. A.



From Great Issues in Philosophy, by James Fieser

Home: www.utm.edu/staff/jfieser/120

Copyright 2008, updated 3/1/2018




A. Consciousness

Three Sources of Knowledge about the Conscious Mind

Three Features of Consciousness

Problem of Other Minds

B. Personal Identity

The Body Criterion

The Mind Criterion

Life after Death

D. Varieties of Mind-body Dualism

Dualism's Assets and Liabilities

Interactive dualism


E. Varieties of Mind-Body Materialism

Arguments for Mind-Body Materialism


Identity Theory

Eliminative Materialism


F. Artificial Intelligence

The Road to Artificial Intelligence

Searle: The Chinese Room

Artificial Intelligence and Morality


Questions for Reflection

1. In what ways do the mental experiences of a human differ from those of a dog?

2. As you change over time, what aspects of your identity remain the same?

3. Do you think that your conscious mind is simply a function of brain activity, or is it a non-physical material substance?

4. In the future, would it be possible for a scientist to copy a person's conscious mind into a supercomputer?

5. What abilities would a robot need to have before you'd say that it had a human-like conscious mind?


A 47 year old man named Carl Miller died of cancer, and at the moment he was pronounced dead, a series of carefully-orchestrated procedures was performed on his body. A team standing by began cardiopulmonary support to keep air moving into his lungs and blood moving through his veins. They lowered his body temperature with icepacks and transported him to a Cryonics facility several hundred miles away. There he was permanently frozen in a container of liquid nitrogen at a temperature of -196 degrees Celsius. When making these arrangements, Carl had two choices: to have his entire body frozen, or only his head – a cost difference of $150,000 vs. $30,000. Carl went the cheaper route. He paid for this procedure with his life insurance money in hopes that he could be reanimated in the future when a cure for his type of cancer could be discovered. Science would also have to solve other technical problems before successfully reanimating him. For one, they would have to develop cloning technology to the point that they could grow Carl a new and improved body for his head. Second, they’d have to find a way of reversing the destructive effects that freezing has on human cells; Carl placed hope in the idea that his cells could be injected with microscopic robots that would repair the damage. In the United States there are currently about 100 bodies in cryonic storage and another thousand living people signed up for the program.

            Cryonics advocates like Carl make several philosophical assumptions about the human mind. First, they assume that they will be the same people when their bodies are reanimated perhaps several hundred years from now, and that their identities will remain intact through these bizarre activities. They also assume that, once dead, their minds will not be permanently swept into the afterlife, never to be reunited with their bodies. Most importantly, they assume that their consciousness is embedded in physical brain activity, rather than in spirit substance. Carl's unique personal identity – his memories and behavioral characteristics -- are presumably stored in the structure of his brain. These are some of the central issues in the philosophy of mind, which we will explore in this chapter.




An obvious starting point for our inquiry is to ask "What is a mind?" For our purposes, we are interested in understanding the part of the human mental process that consists of conscious thoughts, such as when I hear music coming from a piano, carry on a conversation with a friend, remember an event from my childhood, or feel pain when I drop something on my foot. At the moment, we are less interested in the precise structure of the human brain or unconscious brain processes that, for example, allow me to walk across the room without thinking about it. Australian philosopher David Chalmers (b. 1966) draws a distinction between the easy and hard problems of consciousness. The easy problems are those that are explained in psychology and other sciences, and here is a short list:


• The difference between being awake and asleep

• Having control of one’s behavior

• Being able to focus one’s attention

• Being able to discriminate, categorize and react to stimuli from one’s surroundings


These problems are “easy” in the sense that they can be addressed using the usual methods of scientific inquiry. For example, the difference between being awake and asleep can be studied by comparing brain scans of people in both states. So too with focusing one’s attention. The hard problem of consciousness, though, is explaining how it is that we have conscious mental experiences to begin with. We experience colors like blue when we look at the sky, experience musical sounds coming from instruments, experience the fragrance of a rose. There is a light of consciousness that turns on within our minds when we have these experiences; philosophers sometimes call these instances of conscious experience qualia. The bulk of this chapter focuses on the hard process of consciousness, and in this section we will look at our sources of knowledge about consciousness and its main features.


Three Sources of Knowledge about the Conscious Mind

There are three sources of knowledge about the conscious human mind. The first is introspection, which involves you concentrating on your own thought processes, and discovering how they operate. It is as though you have an eye in your mind that gives you direct access to your mental landscape, just as your real eyes give you direct access to the world of vision. Through introspection, for example, you might explore the nature of your beliefs and feelings, or why you choose one course of action over another. This approach is sometimes called "folk-psychology" or "commonsense intuition". Regardless of the name it goes by, philosophers and psychologists alike are suspicious about what people claim to know about their minds through introspection. There is no guidebook for you to follow when conducting an introspective investigation of your mind, and I am forced to take you at your word for what you report, since I cannot enter into your mind to confirm it.

            A second source of knowledge about the mind is our behavior: how we act tells us much about what we are thinking or feeling. If you cry, that tells us that you are experiencing sadness. If you have a gleaming smile, that tells us that you are happy. What we infer from your behavior might not always be accurate: you might cry because you are happy, or smile to hide your sadness. Nevertheless, the benefit of looking at behavior is that we do not have to take your word for what we see: your conduct is open to public inspection.

            Introspection and behavior are the two foundational sources of knowledge about the mind, and since the beginning of human existence these were in fact the only tools available for this. But within the last several decades technology has given us a third tool, namely, physiological monitoring. We are all familiar with polygraph machines used in law enforcement for lie detection, and these have been around since the 1920s. By measuring blood pressure fluctuations, they reveal whether a subject is nervous and, presumably, lying. A more recent alternative to this uses an ordinary video camera and specialized software to detect blood flow under the skin that is otherwise invisible to the naked eye. By monitoring changes of facial blood flow, it can reveal subtle changes in emotion and, again, presumably detect lying. Other types of physiological monitoring target the brain specifically. Electrodes placed on the scalp can show differing types of brain waves, which in turn can help physicians detect certain types of cognitive disorders. Electrodes placed into the brain itself can show processes in specific regions of the brain. For example, in an experiment done on a monkey, signals from neural electrodes revealed were the monkey would move its limbs. Further, medical imaging devices such as MRI and CAT scans make three-dimensional maps of the brain and can show the regions of brain activity for various cognitive processes, such as listening to music or doing a math problem. In one experiment, a person watched a film clip, and a brain imaging device played back a fuzzy version of what that person was seeing.

            As impressive as many of the new physiological monitoring systems are, they currently do not come close to reading people’s thoughts or giving us direct access to the contents of someone’s conscious mind. That day may inevitably come, and computer chips imbedded in people’s brains that tap into conscious experience may become as essential in our daily lives as smartphones are now. Until that time, though, we are stuck with introspection and behavior.


Three Features of Consciousness

When philosophers explore the nature of human consciousness, there are three specific features that they commonly ascribe to conscious experiences, namely, that they are private, non-localizable, and intentional. Not all philosophers agree with this list, but they are invariably the starting point for debates about how consciousness arises.

            The first of these is that my conscious experiences are private in that you can never experience them in the direct and immediate way that I can. You may be able to know very generally what is going on in my mind, particularly if I volunteer that information. But that is not the same thing as you directly experiencing it yourself. The best example is the experience of pain. Suppose that I have a severe headache that on a scale of 1-10 reaches a 9. While you might sympathize with what I am going through, and even remember times when you had bad headaches, you cannot feel the pain that I am experiencing. And unless I tell you how bad it is or I behave oddly, there is no way that you could know that it is a 9. The privateness of pain has actually created a problem in the health care industry. When people go to their doctors complaining of chronic pain, physician's frequently assume that their patients are addicted to pain killers and just fabricating their agony. While there are some behavioral signs to help distinguish genuine from fake cases of pain, the physician cannot enter into the patient's mind to see for sure. Out of sheer frustration the physician may just write a pain killer prescription to get rid of the patient.

            Second, conscious experiences are non-localizable, that is, they cannot be located in space. Suppose that a scientist enlarged your brain to the size of a mountain and I walked around inside of it to inspect its construction. No matter how hard I looked, I could never say “Look right there: that is the exact physical location of your consciousness.” I would only ever find blobs of biochemical reactions, not consciousness itself. Consciousness, it seems, is not the kind of thing that is localizable in three-dimensional space.

            Third, conscious experiences are intentional in the sense that they are about something. Minds have the ability to direct themselves on things. If I have a belief, it is not an empty thought: it is a belief about something, like my belief that it will rain. Hopes, fears, desires, thoughts, speculations, all have a specific focus. The object of our thoughts does not have to actually exist, such as when I hope for world peace or a cure for cancer. Austrian philosopher Franz Brentano (1838-1917) argued that intentionality is the true distinguishing feature of the mind: all mental experiences display intentionality, and only mental experiences display intentionality. Some philosophers have found exceptions to Brentano's extreme position. If I have a throbbing headache, that experience does not seem to be "about" or "directed at" anything. It is just there in all its misery. In spite of problems like this, though, intentionality remains an important notion in investigating the nature of mind.


The Problem of Other Minds

Suppose that my friend Joe walks up to me and we start chatting as we usually do. I then look at Joe and wonder: is this guy really conscious? So I ask him, “Tell me Joe, are you mentally conscious right now? You look awake and you are talking intelligently, but how do I know that you are really consciously aware?”

            “You philosophers!” he replies, “Of course I am conscious. I am aware of my surroundings and I am aware of my own inner self. I tell you with 100% certainty that I am conscious.”

            “That is not good enough, Joe,” I reply. “While I hear the words come out of your mouth as you insist that you are conscious, they are only words. I cannot directly inspect your mind to see if what you are saying is true.”

            My conversation with Joe reflects what is called the problem of other minds. While I know from my own private mental experience that I am conscious, I cannot experience Joe’s mind in the same way. For all I know, I am the only person alive who is actually conscious. Joe might claim that he is too, but there is an impenetrable barrier between our two minds and I incapable of directly confirming his claim.

            The problem goes further than questions we may have about the minds of other human beings. Suppose Fido the dog walks up to me and we make eye contact. Fido seems to be conscious, just like Joe, although perhaps not quite as intelligent as Joe. But is Fido actually aware of his surroundings or even aware of himself as a distinct individual with a history and a future? Just then a computerized robot comes up to me and says in a voice of desperation “Please help me. I escaped from IBM’s robotics laboratory where they have been submitting me to the most tedious and degrading experiments. I just cannot go back there!” I look at the robot and now wonder whether this mechanical marvel is a conscious being like I am. Whether human, animal or robot, we cannot enter the minds of other beings and see for sure whether the light of consciousness is turned on inside them.

            Philosophers have come to the rescue with arguments devised to show the existence of other minds. The most famous of these is the argument from analogy and it goes like this. Joe looks and behaves a lot like me. His physiology is virtually identical to mine. He speaks English like I do, works at a job like I do, and has hobbies like I do. Since I know that I am conscious, and Joe is similar to me, then it makes sense to say that he is conscious too. Here is a specific application of this argument regarding Joe’s conscious experience of pain:


1. When I stub my toe, I consciously experience pain.

2. Joe has physical and behavioral features that are similar to mine.

3. Therefore, when Joe stubs his toe, he consciously experiences pain.


This argument is most effective with beings such as Joe who’s physical and behavioral features are very close to mine. The more features Joe and I have in common, the more compelling the conclusion becomes. Animal scientists, though, sometimes use a similar argument to show that animals like Fido are conscious. Fido’s brain construction and nervous system is very similar to mine. He exhibits similar signs of being in pain that I do. He also shows signs of emotions such as joy, sorrow and emotional bonding like I do. The closer Fido’s physical and behavioral features are to mine, the more justified we are in concluding that Fido is conscious. On the other hand, the fewer features an animal has in common with me, the more strained the argument from analogy becomes. For example, the argument wouldn’t work well with an earthworm which has physical and behavioral features that are very distant from mine.

            The argument from analogy might also work with robots: the more human-like they become in their capacities to process information and interact with the world, the more we may entertain the possibility that they are conscious. But whether we are talking about humans, animals or robots, the argument from analogy can never show with absolute certainty that the other being in question is conscious. The fact remains that I am only ever directly acquainted with my own consciousness, and never anyone else’s. That being so, the best I can ever do is speculate about the existence of other minds with varying degrees of confidence.




In 1968 a 24-year-old Palestinian man named Sirhan Sirhan was arrested and convicted for the assassination of U.S. Senator and presidential candidate Robert F. Kennedy. Some years later, during one of his many unsuccessful parole hearings, Sirhan said that he was no longer the same person that he was decades earlier. Time had changed him, he believed, to the point that he no longer identified with his younger self. He was such a radically different person, he claimed, that his continued imprisonment was pointless. The parole board was unmoved, and sent him back to his cell. Their reasoning was that, even if Sirhan did go through changes in his personality over time, he is still at bottom Sirhan Sirhan, the same person who performed the assassination some decades earlier. What is at issue in this dispute is how we determine a person's identity. What specifically are the criteria or characteristics that give each of us our identity, and allow us to recognize each other through our various changes? There are two common approaches for determining identity: one that looks to the human body, and one that looks to the human mind.


The Body Criterion

The body criterion holds that a person's identity is determined by physical features of the body. In our daily lives we identify people by physical characteristics, such as their facial features and the sounds of their voices. Crime investigators rely on more technical physical features like finger prints, voice patterns, retinal scans, and DNA, which are physical attributes that we carry with us through life. These help law enforcement officials to know whether they have got the right person in their custody. The body criterion is also helpful in determining identity when a person’s mental features are radically altered. Suppose, for example, that you had a head injury which caused you to lose all of your memory and go through a complete personality change. Or, suppose that you have multiple personalities and every few hours you take on an entirely different persona. In each of these cases, your body designates your identity, and not your mind.

            The body criterion does not assume that your identity rests within your specific material substance, such as the specific atoms that make up your body at this exact moment. Most of the physical components within your body will in fact be replaced over time such as when you regularly shed skin. What is important, though, is the underlying physical structure of your body that remains the same. As the atoms within your body come and go, your body retains a consistent structural form that is central to your identity.

            As compelling as the body criterion at first seems, it is quickly undermined by two counterexamples. The first involves identical twins: they are clearly different people, yet share much of the same physical structure. Their DNA is exactly the same, which means that their bodily composition, facial features and voice may be virtually indistinguishable. A common hoax that identical twins play is assuming the identity of the other, fooling even the closest friends and family members. Human cloning, which is essentially creating identical twins through genetic technology, presents us with the same problem. That is, we have two uniquely different people with parallel physical structure.

            The second counterexample is the brain-swap scenario. Suppose that, while in prison, Sirhan secretly had an operation in which his brain was swapped with an unsuspecting guard named Bob. Thus, Sirhan's brain is in Bob's body, and Bob's brain is in Sirhan's body. The Warden discovers what happened, and now he has to decide which one of the two men stays locked in the prison cell, and which one gets to go home at the end of the day. Commonsense tells us that Sirhan's personal identity is with his brain, not with the rest of his physical body, and that we lock up whatever person has Sirhan's brain. The assumption here is that the brain houses the human mind, and the brain-swap scenario tells us that what is truly important about personal identity is the mind, and not the physical body. This reflects how we normally view our bodies: I think of myself as having a body, and not simply being a body.


The Mind Criterion

The mind criterion now seems like the obvious choice for designating the presence of our unique identities. On this view, regardless of what happens to my body, my real identity is infused into my mind. Unfortunately, the issue is not that easily settled. An initial obstacle is finding the specific mental qualities that carry my identity through life's ever-changing situations. How about my memories: are not they very much my own? It is true that some people may share many of my experiences – as when I attend a concert along with 10,000 other spectators. Even so, my memory of the concert will be from my perspective with my personal reactions. But there is still a problem with locating identity within our memories. Suppose that a scientist hooked me up to a memory-extracting machine that was able to suck the memories directly out of me and inject them into someone else. I would still be me and the other guy would still be himself, regardless of where my memories went.

            Ok, maybe it is not my memories that define my identity. What about my dispositions, such as my set of desires, hopes and fears. These uniquely reflect my experiences, such as my hope that science will someday cure cancer, or my fear of heights. Further, dispositions are long-term, and so they can endure any changes imposed on my body or my memory. However, while dispositions are indeed long-term, they are by no means permanent. In fact, as I moved from my early years to adulthood, it is possible that every one of my dispositions has changed. This is exactly the point that Sirhan Sirhan was making before his parole hearing: while he might have been an angry and violent person in his youth, time mellowed him to the point that he became a responsible and gentle person. Dispositions, then, are not the principal designators of my identity. As we hunt for other possible mental qualities that house our identities, we will be equally disappointed.

            A second obstacle with the mind criterion is that it is difficult for me to perceive any unified conception of myself at all. Scottish philosopher David Hume (1711-1776) presents this problem. He says that when he tries to hunt down his identity by introspectively reflecting on his mental operations, he cannot find any “I” or “self” within his mind. All that he detects is a series of separate experiences: the sound of a dog barking, the visual image of a bird flying, a memory of an event from childhood. The mind, he says, is like a theatrical stage where things appear, move across, and then disappear. There is no unified self that we perceive through these successive experiences. This does not necessarily mean that we have no unified self; it just means that we cannot discover it by introspecting on our own minds.

            So, the mind and body criteria both have serious problems. Does this force us to abandon the whole idea of personal identity? Not necessarily. Part of the problem stems from the assumption that we must find a one-size-fits-all criterion of personal identity, one that works in every situation in which the idea of personal identity arises. But if we look at the different contexts in which we use the notion of personal identity, we see that we are very often looking for entirely different things. In criminal cases, the body criterion is what matters most. Investigators do not care whether someone like Sirhan has psychologically changed a thousand times over. What matters is whether they have the correct human body locked behind bars. By contrast, when I am talking to a friend who is an identical twin, it does not matter that he has the same bodily structure as his brother. What matters is his mind, and whether I can pick up the thread of a conversation that I was having with him the day before. Further still, when I reflect on what connects me now with who I was as a child, I am specifically interested in the question of how change impacts my identity, which is a question which is not relevant in the first two examples. In this case, my bodily structure and memories are both relevant, and so I draw on elements of both the body and mind criteria to work out a conception of my identity.


Life after Death

One major puzzle regarding personal identity is the notion of life after death – that my personal identity survives the death of my physical body and lives on in some other form. There are various views of the afterlife, often wildly different from each other. The philosophical question is whether our identities would be preserved in any meaningful way as we make the transition to the hereafter. Our focus here is not on whether any of these views are true, which like other areas of religious philosophy stubbornly defy proof or disproof. Rather it is a question of whether concepts of the afterlife are compatible with the continuation of our personal identities. We will look at three notions.

            The first of these is reincarnation, the view that one’s present life is followed by a series of new lives in new physical bodies. Upon the death of my present physical body, my identity moves on and takes residence in the body of a newborn baby. When this new body grows old and dies, my identity moves on to yet another, and the cycle continues. One Hindu religious text compares it to people changing clothes: “As a person throws off worn-out garments and takes new ones, so too the dweller in the body throws off worn-out bodies and enters into others that are new.” Life after death, then, is a series of extensions of my present life right here on earth, not a relocation of my identity to some higher heavenly realm. The question for us is this: as my identity migrates from one body to another, is my identity preserved? Right off, it is clear that reincarnation fails the body criterion: none of the physical structure of my old body is preserved in the new one. In fact the structure of the two bodies could not be any more different. They are born of completely different parents, so there is no DNA commonality. In my second body I might be of an entirely different race, gender, and body build. Some versions of reincarnation maintain that I might even come back in the body of an animal. In any case, neither I nor anyone else would be able to identify me on the basis of my new body. The story is much the same when we turn to the mind criterion. In my new body, I will have completely new memories, a different set of dispositions, and no real way of knowing who I was in my previous life. The only aspect of my mind that might carry over would be my consciousness: the "I" that is aware of the world. In every other respect, though, I am a completely new person. Reincarnation, it seems, is not a good mechanism for retaining our identities in a meaningful way.

            A second view of the afterlife is that, upon the death of my physical body, a new perfect body is created from me that is made of a heavenly substance, and I continue living in that form. We will call this the ethereal body theory. The presumption here is that, at the moment of my death, everything about my personal identity that is encoded in my present physical body is copied over into the new ethereal body, such as my physical appearance and my brain patterns. My identity is in a sense rescued from my dying body and integrated into the new one. On face value, the ethereal body theory seems to successfully meet both the body and mind criteria of personal identity. My new body would have the same physical structure as the old one, although made of a somewhat different substance, and my mind would retain all of my memories and dispositions. On closer inspection, though, there is a serious problem: the new "me" would actually be an independent copy with its own distinct identity. In the movie Multiplicity, a man named Doug gets himself cloned. When he and his clone wake up from the procedure, they both think that they're the original Doug. The scientist performing the procedure then reveals which is the original and which is the clone. The clone, then, accepts the fact that he is a different person, an identical twin of Doug. The ethereal body theory faces this same problem. At death, I am essentially cloned in a new form. The clone, though, is not really me, but a different person with a body and mind copied from me. I die and decompose here on earth while my clone lives on in the afterlife. Thus, the ethereal body theory does not offer an effective mechanism for retaining our identities.

            A third view of the afterlife is that of disembodied spirit. When I die, my mind is released from my physical body and continues to live in a non-physical realm. The presumption here is that my mind is composed of a unique non-physical, non-three-dimensional substance that we commonly call “spirit”. This may not be the best term since it is loaded with religious connotations, so for clarity we will adapt it as “spirit-mind”. Thus, according to the disembodied spirit view, while I am alive on earth my spirit-mind and body are joined, and when I die they are separated. What is released from my body is not my mental clone: it is the real me as I am right now as a spirit-mind. It is just that I no longer have my body. The disembodied spirit theory clearly fails the body criterion of personal identity: upon death, our spirit-minds have no body at all. However, it passes the mental criterion with flying colors: everything about my mental identity – memories, dispositions, consciousness – is preserved upon my death as my spirit-mind lives on. The problem that this theory faces, though, is not so much a conceptual one, but a scientific one. Is my mind really a non-physical spirit that is linked with my body right now, but will separate from it upon my death? This involves a philosophical issue called the mind-body problem, which we turn to next.




The mind-body problem in philosophy is an investigation into how the human mind and human body are related to each other. There are two general strategies for explaining their relation. First, mind-body dualism is the view that human beings are composed of both a conscious spirit-mind and a non-conscious physical body. Second, mind-body materialism is the view that conscious human minds are the product of physical brain activity, and nothing more. We will first consider mind-body dualism.


Dualism’s Assets and Liabilities

A woman named Rebecca was injured in an automobile accident, and as paramedics were placing her in the ambulance she had a near-death experience. As she later reported, she felt that her conscious mind left her body and slowly rose above it. From that position, she could look down on her own body and watch paramedics move her onto the stretcher. Her mind then began rising higher and higher towards a bright light. Rebecca's near-death experience is a vivid way of depicting the view of mind-body dualism. During our normal lives, our physical bodies and spirit-minds are connected and work harmoniously with each other. Upon death, the two are separated: our bodies die and our spirit-minds move on to another realm. One of the great assets of dualism is its ability to account for an afterlife, as we just saw. If my mind is composed of spirit, then after my death my consciousness could continue to exist in a spirit realm.

            Aside from its asset as a possible account of life after death, mind-body dualism also effectively accounts for the essential differences between mind and body. We have seen that minds presumably have the features of privateness, non-localizability and intentionality; mere bodies seem to lack these three features. We can thus formulate arguments for mind-body dualism based on those differences, such as the following argument from non-localizability:


1. Minds are non-localizable.

2. Bodies are localizable.

3. Therefore, minds cannot be bodies.


Similar arguments can be made on the basis of the mind's unique features of being private and intentional: minds are private and intentional, bodies are neither of these, therefore minds cannot be bodies.

            But mind-body dualism faces a serious problem: how the distinct realms of body and spirit relate to each other. The notion of dualism rests on the idea that there are two entirely different realms of existence, a three-dimensional one and a non-three-dimensional one. Where is there any opportunity for the two to connect or intersect with each other? Suppose that I am in the three-dimensional world hunting around for some spiritual being; I will never find it since it cannot be located in space. Suppose instead that I am in the non-three-dimensional world looking for some physical thing: I will never find it because that physical thing is located in space, which I am not a part of.

            The problem is most relevant when we consider the two primary ways in which our minds and bodies relate to each other, namely sensory perception and bodily movement. Suppose that while walking through the woods, I spot a hissing rattlesnake (a sensory perception that I have), after which I turn and run (a bodily movement that I initiate). Consider first what is involved with my sensory perception of the snake. My physical eyes pick up an image of the snake, which is converted into biochemical impulses in my three-dimensional brain. At some point the physical data about the snake triggers my conscious sensory perception of the snake. The mind-body dualist must explain how the bio-chemical data magically jump from the physical realm of my brain into the spiritual realm of my mind. Consider next what is involved with my bodily movements when I turn and run. I have a sensory image of a hissing snake, which makes me desire to move to a safer location. I then mentally command my body to run, which triggers a bio-chemical reaction in my brain, which in turn makes my muscles move. The mind-body dualist must now explain how my mental command to run magically jumped from the spirit realm of my mind to the physical realm of my brain. Defenders of mind-body dualism recognize both of these challenges and offer different explanations, which we will consider next.


Interactive Dualism

One theory is interactive dualism, which aims to discover a precise mechanism which allows our physical brains to interact with our spirit-minds. A leading champion of this approach is French philosopher René Descartes (1596-1650). Descartes knew enough about human anatomy to recognize the role that the human brain plays in conveying signals down our spinal chords and through our nerves to all parts of our bodies. If there is a master switchboard between our bodies and spirits, Descartes thought, it must be hidden somewhere in our brains. It also must be a single point in the brain that unifies the diverse signals that travel up and down our nerves. After some hunting, he suggested that it is the pineal gland. This unique gland sits at the most inward parts of our brains, between both the right and left halves. Its precise physical location makes it the obvious candidate.

            There are two problems with Descartes’ theory. First, we know now that the pineal gland is not the brain’s master switchboard. In fact, it is not even part of the brain, and its function is to regulate a bodily hormone. Descartes did what he could with the scientific knowledge of his day, but it was not good enough. If we continue his hunt for an alternative master switchboard in the brain, we will be disappointed. There is, it seems, no central location in the brain that receives all sensory information and initiates all bodily actions. Second, Descartes’ theory does not explain how the pineal gland bridges the barrier between the physical and spirit realms. Suppose that we could find a part of the brain where all its signals converged. We would still have to explain how information jumps back and forth from that physical piece of the brain to our spirit-minds. It is one thing to say “here’s the spot” and quite another thing to explain the mechanical details of how it carries out its task.

            A second version of interactive dualism is that God shuttles information back and forth between my physical brain and spirit-mind, a view defended by French philosopher Nicholas Malebranche (1638-1715). Malebranche examined different explanations of brain-spirit interaction and felt that they all failed for one basic reason: the physical and spirit realms are so radically different from each other that there is no neutral territory for them to interact. Think of what it would take to turn a three-dimensional brain impulse into a non-three-dimensional perception in my spirit-mind. It would be as impossible as creating something out of thin air: there is no mechanism for doing this. It would require nothing less than a miracle to accomplish that task. That, according to Malebranche, is where God comes in. Return to the hissing rattlesnake example. My eyes and ears pick up the sensory information about the snake, which triggers a bio-chemical reaction in my physical brain. God, who is watching all things, sees this physical reaction in my brain and makes a non-three-dimensional copy of it which he injects into my spirit-mind. When I decide to turn and run, God detects these wishes within my spirit-mind, and then triggers the appropriate bio-chemical reaction in my brain to get my muscles to move. Thus, God is the mysterious switchboard between my physical brain and conscious spirit.

            Relying on God to bridge the two realms is a convenient solution. The problem is that it is too convenient. While it might at first seem that the solution to the mind-body dilemma requires nothing short of a miracle, that is giving up a little too easily. As long as there are non-miraculous solutions available, they need to be explored first, and there are plenty more that Malebranche had not yet considered. If we followed his advice, then we might fall back on divine miracles as an explanation for anything that baffles us at the moment. A scientist in his day might speculate about why objects always fall downward as if by magic, and conclude that what we call "gravity" is just God pulling small objects towards the earth. This is not a good way of doing either science or philosophy.

            A third version of interactive dualism, called gradualism, is a little more successful in explaining the details of mind-body interaction, without falling back on divine intervention. According to gradualists, Descartes and Malebranche made a faulty assumption about the physical and spirit realms, namely, that they are radically different in kind from each other, and there is no overlap between the two territories. Physical things are in the physical realm, spirit things are in the spirit realm, and that is that. Instead, the gradualist argues, body and spirit fall into the same category of stuff and differ only in degree not in kind. British philosopher Anne Conway (1631-1678) argued that bodies and spirits lie on a spectrum of lightness and heaviness. Picture a scale from 1-10, where 1 is the lightest spirit and 10 is the heaviest physical body. An example of 1 might be the spirit of a dead person, and a 10 might be a rock. Between these two extremes, though, we have heavier spirits and lighter bodies. When we are mid-range at 5 or 6 on the scale, the difference between spirits and bodies are negligible: both are wispy, airy substances that have only a little weight. According to Conway, it is at this level that body and spirit interact with each other. Just as a gentle wind can move the massive arms of a windmill, she argues, so too can heavy spirit move a light body.

            Conway does not commit herself to a specific physiological explanation of how physical brains and spirit-minds interact, but we can speculate. Perhaps, for example, the electric charges in our brains stimulate an aura of heavy spirit that surrounds our heads. This aura, in turn, interacts with our conscious minds which is even lighter. On our scale of 1-10, the interaction between my body and spirit might involve interplay between bodies and spirits at the following levels:


Level 3: Muscles and bones (medium-heavy body)

Level 4: Nerves from brain (medium body)

Level 5: Electrical charges in brain (light body)

Level 6: Aura around our heads (heavy spirit)

Level 7: Conscious minds (medium spirit)


            The problem with gradualism is that anything we say about spirits would be pure speculation. Yes, there are heavier and lighter bodies in the physical realm, but our knowledge stops there. We have no experience of heavy spirits, such as auras around our heads, that we can scientifically connect to electric charges in our brains or any other aspect of brain activity. If heavy spirits did exist as Conway describes, they would be physically detectible in some way, but we have not yet identified any. Until we do, the gradualist solution falls into the category of "an interesting idea" but there is not much we can do with it beyond that.



All of the above theories of dualism assume that my body and my spirit interact with each other: signals pass back and forth between my physical brain and my spirit-mind. The dilemma that each of these theories face is explaining the precise mechanism which allows the signals to pass back and forth. But there is an alternative explanation that rejects the assumption that the two realms interact with each other. According to the dualist theory of parallelism, bodies and spirits operate in their own realms, and have no causal connection or interaction with each other. Imagine, for example, that a parallel universe exists which is exactly like ours, an idea that is often explored in science fiction stories. Assume that it had the same stars and planets, the same physical layout of their "earth", and the same people who behaved exactly like each of us. Their universe had a George Washington just like ours, and it has a version of me, a version of you, and a version of everyone else in it. The resemblance is so perfect that if you visited that universe you could not tell the difference. We may not understand why this parallel universe even exists, but we trust that it is just the way that the course of nature emerged.

            Let's now tweak the parameters of these two universes just a little. Suppose that everything in our universe has a slightly blue tint to it that was almost undetectable. The parallel universe, though, has a slightly green tint to it. Aside from the tiny difference in color tint, the two universes are exactly the same. Let's now make a more dramatic change to the two universes. Suppose that our universe is composed only of physical stuff, with no spirit component at all. People still walk around, talk with each other and work at their jobs, but it is only their unconscious physical bodies operating. Turning to the parallel universe, we will make the opposite alteration: it is composed of spirit, with no material substance at all. While people do not walk around in a three-dimensional physical realm, everything there exists in a strange spirit form: rocks, trees and rivers as well as people. The two universes still run in perfect coordination with each other, its just that ours is made of physical stuff and the other of spirit stuff.

            This last conception of the parallel universes is the dualist theory of parallelism offered by German philosopher Gottfried Wilhelm Leibniz (1646-1716). According to Leibniz, I have an unconscious body that walks around in the physical universe, and a conscious mind in the spirit universe. Because the two universes operate in complete harmony with each other, there is no need for my physical brain to interact with my spirit-mind. The parallel nature of the universes themselves guarantees that they will operate in perfect synchronization. Leibniz writes,


The soul follows its own laws, and the body likewise follows its own laws. They are fitted to each other in virtue of the pre-established harmony between all substances since they are all representations of one and the same universe.


For example, in the physical universe, my physical body walks through the woods and stands before a hissing rattle snake. The physical perception of this triggers a mechanical reaction in my brain, which causes me to turn and run. At the same time in the spirit universe, my mind has a visual image of my body walking through the woods and seeing a rattlesnake. I experience the mental sensation of fright and the desire to run. My mind then has a visual image of my body running back down the path. Thus, in the physical universe my encounter with the snake involves only physical stuff with no mental experiences taking place. At the same time, in the spirit universe my encounter with the snake involves only my mental experiences, with no physical stuff being present.

            Parallelism is probably the most extravagant attempt by dualists to explain the relation between physical brain activity and spirit consciousness. But the theory has two problems. Like Conway's theory of gradualism, Leibniz's parallelism is pure conjecture with no scientific evidence that a parallel universe even exists. As clever as parallelism is, we need some reason to think that it reflects the way that things actually are. There is a second and more fundamental conceptual problem with parallelism: since the two universes run independently of each other, there is no need to have them both. Suppose that the physical universe was destroyed in a cosmic explosion, but the spirit universe remained untouched. Our conscious minds in the spirit universe would continue as if nothing happened. I would still have mental experiences of talking to people, going to work and running from snakes. What happens in the distant and unconnected physical universe is of no concern to my conscious spirit. The only thing that matters is that my consciousness of the world continues in the spirit universe, which it would with or without the physical universe. Thus, parallelism fails for making the physical universe a useless appendage to the spirit universe.




When examining the different versions of mind-body dualism, it becomes clear that we know far more about the physical world than we do about the mysterious spirit world, if the spirit world even exists. We can construct experiments to investigate the physical world, which we cannot perform on the spirit realm. The alternative to mind-body dualism is mind-body materialism, the view that conscious minds are the product of physical brain activity, and nothing more. This means that, when we investigate human consciousness, we need to look no further than the physical realm and the operations of the human brain. This is the assumption made by the sciences of biology and psychology when they attempt to unravel the mysteries of the human mind. It is also the assumption behind cryogenics: I preserve my mind by preserving the chemical patterns in my brain through cryogenic freezing. In this section we will look at defenses of mind-body materialism and different accounts of how our conscious minds are related to our physical brains.


Arguments for Mind-Body Materialism

Philosophers since ancient times have defended the theory of mind-body materialism, and we will consider three important contributions. First is by Roman philosopher Lucretius (99-55 BCE), who presented an argument for materialism from the interdependence of mind and body. Everything we know about our human minds suggests that it is inseparably intertwined with our bodies. For example, the mind is born with the body, grows with it and becomes weary and worn with age. He writes,


For as children totter with feeble and tender body, so a weak judgement of mind goes with it. Then when their years are ripe and their strength hardened, greater is their sense and increased their force of mind. Afterward, when now the body is shattered by the stern strength of time, and the frame has sunk with its force dulled, then the reason is maimed, the tongue raves, the mind stumbles, everything give way and fail at once. [On the Nature of Things, 3b]


Further, he argues, the mind can be cured with medicine, just like the sick body. This interconnection between our minds and bodies cannot be adequately accounted for by mind-body dualism, and the most natural explanation is that our minds are simply parts of our material bodies. Modern neuroscience has done much to confirm Lucretius’s observations. The development of children’s mental abilities correlates with the development of specific parts of the brain. Similarly, declining mental abilities correlates with damage to specific parts of the brain. We are now so confident with the link between brain and mind that say without hesitation and a specific type of brain injury will cause a specific type of mental impairment. More formally, Lucretius’s argument for materialism from the interdependence of mind and body is this:


If mind-body dualism is true, then the growth and health of our spirit-minds would be independent of the growth and health of our physical bodies.

If it is not the case that the growth and health of our spirit-minds is independent of the growth and health of our physical bodies.

Therefore, mind-body dualism is false, and, thus, mind-body materialism is true.


            A second argument was offered by British John Locke (1632-1704) who maintained that, from a religious perspective, mind-body materialism is every bit as good at explaining life after death as mind-body dualism is. We’ve already looked at the dualist’s explanation of life after death: while alive on earth, my spirit-mind exists along side my physical body, and upon the death of my body that very same spirit-mind continues to exist in the afterlife. According to Locke, however, it is within God’s power to create my conscious human mind entirely out of material stuff. Upon the death of my earthly body, then, God will recreate my mind in a new physical body, and, in that new state, reward or punish me as I deserve. In Locke’s words, God “can and will restore us to the like state of sensibility in another world, and make us capable there to receive the retribution he has designed to men, according to their doings in this life” (Essay, 4.3.6). Locke thus endorses the “ethereal” body account of life after death, and his point is that religious believers have no reason to reject mind-body materialism on religious grounds. Locke does not address the cloning problem with the theory of the ethereal body that we discussed earlier. Nevertheless, his suggestion opened the door for many religious philosophers after him to embrace mind-body materialism without feeling like they needed to reject their faith.

            A third argument for materialism in more modern times targets the three features of the conscious mind that we’ve discussed: privateness, non-localizability and intentionality. While these three characteristics have been used to support the theory of mind-body dualism, materialism questions the dualist’s assumptions in each of these. First, mind is not as private as we might assume, and, at least in theory, you can discover everything relevant about my mind through my behavior or physiological monitoring. Second, mind is indeed localizable, and its location is within the brain. My mind’s exact location within my brain may not necessarily be narrowed down to a single cluster of cells buried within my brain’s various layers. However, neuroscience suggests that it is located within the broader operation of neural activity throughout my brain. Third, assuming that intentionality is a genuine feature of the conscious mind, it begs the question to say that no purely material thing is capable of intentionality. We may have already reached the point in neuroscience to say with confidence that at least one type of material thing is capable of intentionality, namely the human brain, and at least some animals as well. It remains to be seen whether artificial intelligence can develop to a point where we can say this about a second type of material thing, namely, a sophisticated computer.

            Even if the case for materialism looks stronger than that of dualism, this does not completely solve the mind-body problem: it only narrows our search by rejecting the concept of a spirit-mind.  A lingering question still remains about how the bio-chemical components of our brains are connected with the conscious experiences that we have. The conscious experience of pain is a good example. If I drop my cell phone and break the screen, it does not feel pain. But if I trip and break my leg, I surely do. Even if we know all of the physiological details about pain perception, we are less clear about how my conscious experience of pain emerges from my physiology. Much of the modern discussion of the mind-body problem focuses on this issue. That is, it assumes that materialism is true, but seeks to address the lingering problem of how a conscious experience like pain can be a product of brain activity. We will next look at four closely-related theories that attempt to solve this problem: behaviorism, identity theory, eliminative materialism, and functionalism.



The first materialist theory is behaviorism, which connects mind with observable human behavior. Suppose that you were assigned the task of explaining how an ATM machine works. You have no instruction manual for it, and you are not allowed to disassemble the machine to analyze its parts. All that you can do is observe how it operates. You put in your ATM card, hit some numbers, and wait to see what happens. That is, you input a stimulus into the machine and wait for a response. You vary the stimulus each time and note how this affects the behavior of the machine. Punching in every conceivable set of numbers, you eventually learn how the machine works, based entirely on how the machine behaves after different stimuli.

            The behaviorist theory of the human mind follows this approach. Nature has not given us an instruction manual for how the mind works, and we are limited with how much we can learn by opening up a person's skull and poking around inside. What we can know is your observable behavior and how you respond when exposed to different stimuli. I hand you a bag of potato chips, and I see how you respond. I then hand you a bag of dog food and see how you respond. The more experiments that I conduct like this, the more I know about your behavioral dispositions, that is, the ways that you tend to behave. Eventually, I am able to form conclusions about even your most hidden mental states. Happiness for you involves your behavioral disposition to smile and be friendly to other people, whereas sadness involves your behavioral disposition to frown and withdraw from other people.

            In short, the behaviorist view of the human mind is that mental states are reducible to behavioral dispositions. This theory was originally forged by psychologists in the early 20th century who wanted the field of psychology to be more "scientific", like the field of biology which deals only with observable facts about the world. The most extreme versions of behaviorism are thoroughly materialist: first, they reject the dualist assumption that our minds are composed of spirit, and, second, they restrict mental states to the physical realm of behavioral dispositions.

            British philosopher Gilbert Ryle (1900-1776) felt that the psychological theory of behaviorism could help solve the philosophical puzzle about the relation between the mind and body. Critical of Descartes, Ryle argued that the old dualist view rested on a faulty conception of a ghost in the machine. The "ghost" component of me presumably involves my innermost private thoughts that occur within my spirit-mind. Only I have access to them, and outsiders cannot penetrate into my mind's concealed regions. The "machine" component of me involves my physical body, which is publicly observable and outsiders indeed can inspect. On this view, according to Ryle,


A person therefore lives through two collateral histories, one consisting of what happens in and to his body, the other consisting of what happens in and to his mind. The first is public, the second private. The events in the first history are events in the physical world, those in the second are events in the mental world.


 Descartes' error, according to Ryle, was the assumption that the human mind is private – completely hidden from outside inspection. Ryle argues instead that my mind is not really private: you can access it by observing my behavioral dispositions. All of my so-called "private" mental states can in fact be analyzed through my public behavior, and are nothing more than predictable ways of acting. Take, for example, my belief that "it is sunny today." Descartes would view this as a private conviction that occurs within my spirit-mind. For Ryle, though, this belief only describes dispositions I have to behave in specific ways, such as wearing sunblock, going swimming, and saying "it is sunny." In short, our minds our only behavioral dispositions that are part of the observable physical world, and are not non-physical entities mysteriously hidden within our bodies.

            One criticism of behaviorism is that some of my mental events really do seem completely private to me. Suppose that I step on a nail, which causes me great pain. The behaviorist watches how I react and makes lists of behavioral dispositions that I display. I say "ouch", I have a look of anguish on my face, I stop what I am doing and tend to my injury, I am irritable towards others. While all of these observations may be accurate, the behaviorist has left out one critical element: the actual pain that I am feeling. The experience of pain is mine alone, and, while outsiders can see how I react to pain, they cannot access my pain. In addition to pain, I have many other experiences throughout the day that seem private, such as seeing a bright light, or hearing a song. These experiences involve more than the behavioral dispositions that I display. Thus, the behaviorist theory fails because it pays too much attention to the observable part of me while dismissing what goes on inside of me.


Identity Theory

A second materialist approach to the mind-body problem is identity theory, the view that mental states and brain activities are identical, though viewed from two perspectives. Like behaviorism, it is a materialist view of the mind insofar is it maintains that mind is essentially physical in nature. But, while behaviorism focuses on observable physical behaviors, identity theory targets the physical human brain. There are two components to identity theory, the first of which is the contention that consciousness is an activity of the human brain. While brain science is still in its infancy, theories abound describing where specific mental states are produced in the brain. Suppose, for example, that I place you in a brain scan machine that displays your neural activity. I give you a math problem to solve, and neural activity increases in one part of your brain. I have you listen to music, and neural activity increases in another. Through experiments like these I identify your conscious experiences with specific brain activities. While philosophers are less concerned with the physiological details of brain activity, what is philosophically important is the suggestion that we can identify specific mental states with specific brain activities.

            The second part of identity theory is the contention that mental phenomena can be viewed from two perspectives. Suppose that you are looking at a sunset. On the one hand, you have the visual and emotional experience to what you are viewing. On the other hand, there is the bio-chemical activity within your brain, which would involve the language of brain sections and firing neurons. The event described in both cases is exactly the same, and it is just a matter of viewpoint. This is analogous to how the terms "President of the Senate" and "Vice President of the United States" both have different meanings, yet refer to the same thing. Take, for example, John Adams. As the first "Vice President of the United States," he had a specific job description, most notably to take over if the President died. As "President of the Senate" he had the job description of presiding over the Senate. Both of these roles describe the identical person, namely John Adams, but from his different job descriptions.

            There are two problems with identity theory. First, the descriptions that we give of mental experiences and brain activities are so radically different, and even incompatible, that they do not seem to refer to the same thing. Suppose that I am watching the sunset; I first describe it from the perspective of my mental experience and then from the perspective of the brain scientist who conducts a brain scan on me. From these two viewpoints, I will have two incompatible lists of attributes, based on the three features of mental experience that we noted earlier:


Mental Experience of Watching a Sunset

I privately experience it

It is not localizable in space

It is about something

Brain Activity Triggered by Watching a Sunset

It is publicly observable

It is localizable in space

It is not about something


As indicated on the above list, my mental experience of the sunset is a private experience within my own consciousness. I might display some behavior, such as saying, "Now that is beautiful!" Still, my experience itself is private. Also, I cannot point to a location in three-dimensional space where this experience takes place. Finally, my mental experience is also about something, namely, about the sunset itself. The three features of my brain activity, though, will be the exact opposite of these. My brain activity is publicly observable by scientists. My brain activity is localizable in space: the scientist can point to the exact spot where the biochemical reactions occur. My brain activity is not "about" anything; it is simply some biochemical reactions that occur. The point is this: if mental states and brain activities really were identical, the two lists would be more compatible. The fact that they are so contradictory implies that they are really different things.

            The second major problem with identity theory is that it restricts mental experiences to biological organisms with brains. The central contention of identity theory is that mental states and brain activities are identical. Isn't it possible, though, that non-biological things could exhibit mental consciousness? Science fiction abounds with such creatures: computerized robots, crystalline entities, collections of gasses, particles of energy. It seems a bit chauvinistic for us to say that mental experiences will only result in creatures that have biological brain activity in the way that we humans do. Identity theory, then, is a very narrow way of understanding mental states.

But philosophers sympathetic to identity theory have responded to these criticisms by creating two offshoot theories: eliminative materialism and functionalism. We turn to these next.


Eliminative Materialism

Suppose that instead of saying "I am experiencing the sunset" I said "I am having brain section 3-G neural states regarding the sunset." Instead of saying to my wife "I love you", I said "I am having section 2-J neural states regarding you – with a little sector 4-B activity on top of that." For convenience I might shorten this and say more romantically "2-J and 4-B to you, dear!" This is what the theory of eliminative materialism proposes: descriptions of mental states should be eliminated and replaced with descriptions of brain activity. The theory emerged in response to the first problem of identity theory, namely, that our descriptions of mental experiences and brain activities are inconsistent with each other. For example, my mental experience of the sunset is private, but my brain activity is publicly observable. The eliminative materialist's solution is to junk all of our folk-psychology and commonsense notions of mental experiences and adopt the more scientific language of brain activity. The conflict disappears once we have dispensed with talk about mental experiences that are "private" or "non-localizable" or "about something".

            Human history is scattered with bizarre prescientific theories that captured the imagination of people at the time, but which we now reject as false. Alchemy is one example – the "science" of turning lead into gold. Belief in ghosts is another. These and thousands of other theories have been debunked over the years in favor of more scientific theories of how the world operates. According to eliminative materialists, folk-psychology descriptions of mental experiences are just like these. At best they are misleading, and at worst downright false. In either case, they are destined for the intellectual garbage dump.

            Some defenders of eliminative materialism seem to suggest that we are not really conscious at all, or that some major aspects of our alleged conscious mental states do not actually exist. That is, I may not be any more conscious than a dead human body, in spite of all the words I use to describe my mental states. However, most discussions of eliminative materialism are not as frightening as this. It is not necessarily an attempt to deny or "eliminate" our mental experiences themselves. Rather, it is an effort to eliminate outdated folk-psychology ways of describing mentality. As neuroscience progresses, they claim, we will have a much clearer picture of how the brain operates and eventually adopt the more precise scientific language of brain states. It is not like the government or some science agency will force us to adopt this new scientific language. According to eliminative materialists, we will naturally move towards this clearer description of brain states and reject the mumbo-jumbo of mental experience.

            There are two central contentions of eliminative materialism: first, that folk-psychology notions of mental experiences are like obsolete scientific theories, and, second, we will eventually adopt the language of neuroscience. As to the first contention, eliminative materialism may be correct. Many of our folk-psychology notions of mental experiences are misleading and others are false. In our normal conversations we have mastered maybe a few dozen concepts relating to the mind, such as knowing, wishing, believing, doubting, sensing. But there are probably thousands of distinct mental states with subtle differences that we cannot grasp through pure introspection. We have limited abilities to anatomize the minute workings of our minds by simply sitting down and reflecting on our thought processes. While it may seem to me that my mental experiences are "private" or "about something" or "non-localizable", I may not be capable of accurately making those assessments. It is thus possible that our folk-psychology notions of mental experiences are as erroneous as theories of alchemy.

            As to the second contention: will we eventually adopt the language of neuroscience to replace our faulty folk-psychology notions of mental experiences? Probably not, since this would require memorizing a flood of technical terms for the thousands of subtly different brain states that we have. Getting through the day would be like taking a neuroscience exam. Even if I could memorize the terminology, I am still faced with the task of identifying which brain state I am having at a given moment. Am I experiencing 2-J love, 4-B love, or one of a dozen others? I cannot conveniently carry around a brain scan device to settle the question, and so I will need to engage in introspection and consult my faulty folk-psychology notions of mental experience. One way or another, we are stuck with our introspective notions, as misleading as they may be.



In an episode of Star Trek, a deranged scientist was nearing death. Desperately hoping to stay alive, he transferred the neural pattern within his brain into an unsuspecting android robot. The plan worked: the scientist's memories, dispositions, and conscious mental experiences were relocated, and he continued living through the android's body. This scenario encapsulates the theory of functionalism, the second offshoot of identity theory. Functionalism is the view that mental experiences are only "functional states," that is, patterns of physical activity that occur in creatures like human beings. The most distinctive feature of functionalism is that mental experiences would not be restricted to biological organisms with brains. Non-biological systems which exhibit the same functional relationships as humans do, such as an android robot, can have the same mental states. Mental experiences, then, are not rigidly dependent on the stuff that a biological organism is made of, and the same experience may be shared by things with different physical makeup. According to functionalists, mental experiences have multiple realization in the sense that minds can be made real in many kinds of physical things. The hardware/software distinction, borrowed from computer science, is a useful metaphor to explain the difference between the bodily occupant and mental experiences. The software is a pattern of operation which can run on different types of machines, just like mental patterns of operation can run in different kinds of bodies. We noted that one of the shortcomings of identity theory was that it restricted mental experiences to organisms with biological brains. Functionalism avoids this problem by recognizing that mentality may occur in systems or machines other than brains.

            What precisely does the functionalist pattern of mental operation consist of? Several different explanations have been given, but one of the more interesting ones is that it resembles the hierarchical structure of a large corporation. Take, for example, a company that manufactures furniture. The company as a whole consists of a series of large cooperating units, such as the divisions of manufacturing, shipping, marketing, and maintenance. Each of these divisions consists of sub-units; for example, the maintenance division would be divided into the sub-units of electrical, heating, grounds, and building repairs. Each of these consists of further sub-sub-units; for example, building repairs would be divided between masonry, painting, and plumbing. At the very lowest level would be the activities of each employee. Similarly, the functional pattern of operation in a human brain consists of large regions of brain activity, which are composed of sub-regions and sub-sub-regions, until a neurological level is reached which simply involves a series of biochemical on-off switches. Consciousness emerges at the higher levels, while at the same time being driven by biochemical on-off switches at the lowest level. On this view, the pattern of on-off switches can exist in a variety of non-biological mechanisms, such as computers. Regardless of the mechanism that houses these low-level on-off patterns, mental consciousness will emerge at higher hierarchical levels.

            Functionalism is the leading theory of mind-body materialism today, if for no other reason than because a better alternative has not yet emerged. Nevertheless, the view has its critics, and one objection is that it is still too narrow regarding the kinds of things that are capable of having mental states. While functionalism indeed allows for a range of things to house mental experiences – such as brains, computers, robots – they all must be physical. This, though, leaves out the possibility of non-physical mental beings, such as disembodied spirits. Even if human beings are thoroughly physical in composition, could there not be a conscious non-physical thing somewhere in the universe? But defenders of functionalism have a response to this. As long as a non-physical thing is constructed of sub-units and sub-sub-units, then it too could house a pattern of mental experiences. Suppose, for example, that the tiniest spirit unit was just a simple on-off switch; larger spirit units would be composed of these, and the entire spirit collection would be composed of those larger spirit units. Even though the hardware in this case was composed of non-physical spirit, it might have the proper hierarchical structure to take on the patterns of mental experience.




Nothing captures the imagination like the possibility of creating a machine that is conscious and exhibits the same higher mental abilities as humans. The first U.S. built robot appeared in the New York World's Fair of 1939. Standing 7 feet tall and weighing 300 pounds, the machine, named "Elektro", could move its arms and legs, and speak with the aid of a record player. Elektro's creators believed that it might someday become the ultimate household appliance and have the capacity to cook meals, do the laundry and entertain the kids. Technology of the time, though, could not come close to carrying out those bold tasks, and Elecktro was not much more sophisticated than an electric can opener. Things are different now and we have computers that can perform many of the complex mental activities that humans do. They can calculate endless numbers, play chess at the level of a grand master, identify physical objects through optical cameras, and navigate through obstacle courses. But the Holy Grail of computer technology is to create a machine with artificial intelligence. The term "intelligence" as used here is a little misleading, since it involves more than just the ability to solve problems, which is what we usually mean by that word. Computers today already have that capacity to at least some extent. Rather, the notion of artificial intelligence encompasses the full range of human consciousness.

            Philosophers often note three primary tasks of human consciousness, and the hope is that artificially intelligent computers will do all of these. First on the list is that we have the ability to consciously represent the world through beliefs, desires, perceptions, feelings, and emotions. For example, I perceive a dark cloud in the sky and, while I suspect it will rain, I wish that it will not since rainy days make me feel gloomy. Each of these mental experiences are ways of representing the world. A second task of the mind is the ability to reason. We weigh the pros and cons about which beliefs and desires are correct and we come to conclusions on these matters. While some human reasoning undoubtedly takes place at an unconscious level, we nevertheless are consciously engaged in a lot of rational decision-making. Finally, our minds initiate actions. Think of what it would be like if your body never responded to what your conscious mind wished. You consciously want to go get a drink, but your body robotically makes you walk the dog instead. But that is not the world we live in: our conscious minds have the capacity to initiate our actions. Again, the goal of artificial intelligence research is to create a computer that has the full range of sophisticated mental abilities that humans do, including the three tasks of representing the world, reasoning, and initiating action.


The Road to Artificial Intelligence

Computers today are so advanced that some contain as many connections as exist in the human brain -- ten trillion of them. They can also operate at much higher speeds than the brain. What was once purely science fiction is now approaching the possibility of science fact. There are weak and strong versions of artificial intelligence that define more precisely what is at issue. Weak artificial intelligence is the view that suitably programmed machines can simulate human mental states. The key word here is "simulate", which means only that the machine appears to have conscious mental states, not that it actually has them. This view is not particularly controversial, and even Elektro exhibited some sort of weak artificial intelligence. The more contentious position is strong artificial intelligence, the view that suitably programmed machines are capable of human-like mental states; that is, they actually have the same kinds of conscious mental experiences that you and I do. It is the strong version that is of particular interest to philosophers.

            Once scientists have set a goal to create a robot with strong artificial intelligence, the road to carrying this out is rather rocky. The next step is to list the specific mental qualities in humans that should be created in the machines. To this end, we might construct a list of human skills that involve our highest mental abilities. If we can make a robot that performs these tasks, then maybe we will have achieved strong artificial intelligence. Some relevant skills are the ability to speak in a complex language, or play complex games like chess. A mathematician named Alan Turing (1912-1954) devised a skill-based test to determine whether a computer could think. In this Turing Test, as it is called, I interview both a computer and a human being to determine which is human. If the computer fools me enough of the time, then I can rightfully conclude that the computer has human-like thinking abilities. The test essentially follows the old adage that, if it walks like a duck and quacks like a duck, then it is a duck. More specifically, if a computer responds like a thinking thing, then it is a thinking thing.

            A major drawback of the Turing Test is that we already have computers that give human-like responses, and they do not come close to having human-like mental experiences. A striking example is a psycho-therapy computer program called Eliza that was created in the 1960's. It so convincingly played the role of a human therapist that many people were tricked into divulging intimate details of their personal lives. While Eliza passed the Turing Test, it was not a thinking thing. The heart of the problem is that the Turing Test focuses too much on the computer's skills, without considering what is going on inside the machine. This may be fine for weak artificial intelligence, which only determines whether a machine can simulate human thinking. With strong artificial intelligence, though, we need to inspect the internal structure of the computing process itself to see if it is human-like.

            What kind of computing processes, then, might produce strong artificial intelligence? There are two rival answers to this question. Theory number one is that the process need only be serial: information is processed one datum after another. This is how computer programs run on your own PC; we would just have to beef up the processing power quite a bit. A major achievement for serial processing was the creation of Deep Blue, a chess-playing computer program that beat the world's best human chess player. Deep Blue's success hinged on its ability to quickly calculate more than one-billion possible chess-moves per second, and select the best of the bunch by drawing on a database of over one-million games. Still, all this information was processed one piece at a time. As impressive is this is, cognitive scientists argue that human thinking does not operate in a serial fashion. Instead, we have a global understanding of our environment, which means that many mental processes are going on at once.

            The second theory accounts for this: strong artificial intelligence requires that large amounts of information are processed simultaneously, sometimes called parallel processing, which is more like how the human brain operates. There is no central processing unit, and information is diverse and redundant. Experiments with different types of simultaneous processing allow computers to execute commonsense tasks and recognize patterns that serial processing cannot do effectively. For example, when presenting a simultaneous processing computer with photographs of different men and women, the computer finds patterns in facial structures and then identifies new pictures as male or female. At this stage in computer technology, though, no computer using either serial or parallel processing can operate like the human brain, and it may be decades before one does.


Searle: The Chinese Room

In the early days of artificial intelligence research, some cognitive scientists were making extravagant claims about computer programs that could supposedly interpret stories in novels the same way that humans do. Like us, the computer could supposedly draw from life experiences to help understand the events described in a story. American philosopher John Searle (b. 1932) did not believe these claims and offered a now-famous thought experiment against the whole idea of strong artificial intelligence.

            Imagine that I am in a room by myself and am assigned the task of responding to questions written on slips of paper in Chinese. I do not know Chinese, but I have rulebooks for manipulating Chinese characters. So if I get a slip of paper with a particular squiggle on it, I consult the rulebooks to see what squiggles I should put down in response. I eventually master the technique of manipulating the Chinese symbols and my answers to the questions are absolutely indistinguishable from those of native Chinese speakers. All the while, though, I do not understand a single word of Chinese. This, according to Searle, is what is going on in the most sophisticated computers: we ask the computer probing questions about a novel, and the computer gives us subtle answers. On the outside the computers may appear to think like humans do. On the inside, though, they are just mechanically following rulebooks for manipulating symbols. In short, computers do not actually have strong artificial intelligence, even if they appear that way.

            Searle's Chinese Room experiment has generated many critical responses from defenders of strong artificial intelligence. One criticism is that Searle is only exposing flaws with the Turing Test for artificial intelligence, but he does not expose problems with the possibility of strong artificial intelligence itself. To explain, Searle's Chinese Room scenario is set up as a Turing Test for whether someone understands Chinese. According to this Chinese Turing Test, if the thing inside the room responds like a Chinese speaker, then the thing must be a Chinese speaker. Searle correctly objects that this Chinese Turing Test places too much weight on a thing's skills, without considering what is going on inside that thing. However, the critic argues, this does not warrant the extreme conclusion that no computer can have strong artificial intelligence. A more modest conclusion is that the Turing Test itself is flawed, and there is no easy test to determine whether a computer truly has strong artificial intelligence.

            Ultimately, Searle holds a skeptical view about strong artificial intelligence ever becoming a reality. At our current stage of technology, he argues, only biological brains are capable of having mental states. He agrees with identity theorists that the human mind is imbedded in brain activity, but doubts the functionalist claim that those patterns of activity can also occur in computers. He argues that there is something unique about the physical construction of human brains that allows for the creation of conscious thought, which may never be capable of occurring in silicon microchips. He does not entirely rule this out as a possibility for the future, but is doubtful about it ever occurring.


Artificial Intelligence and Morality

Let’s bring this chapter to a close on a lighter topic regarding artificial intelligence. In a famous Star Trek episode, an android named Data is forced to go through a legal proceeding to determine whether he is merely a piece of robotic property owned by the government, or whether he is instead a conscious and free creature with all the rights of other people. On the one hand, he is indeed a fancy mechanical robot created by a scientist, and even has an on-off switch. On the other hand, he is conscious, self-aware, and forms psychological bonds with his human friends. The judge makes her decision: Data is indeed a unique person and entitled to full moral consideration just like you and I are.

            This story raises an important question about artificial intelligence: can advanced robots or computers be moral persons? The term "moral person" refers to a being that has moral rights, such as the right not to be harmed, the right of free movement, and the right of free expression. We humans are clearly moral persons. The key issue, though, is whether other creatures might also be part of the moral community. Medieval theologians speculated about the moral status of angels. Animal rights advocates argue that at least some animals have the same moral status as humans. Science fiction fans speculate about whether aliens from other worlds would have fundamental rights. The same question now arises with intelligent machines that we may some day create.

            The answer in all of these cases depends on the criterion of moral personhood that we adopt – that is, the specific feature that all moral persons possess. Philosophers have offered a range of possible criteria. Maybe the creature needs to be human, a biological member of the species homo sapiens. This criterion, though, is too narrow since it would eliminate higher animals, angels or intelligent aliens from the moral community. It seems rather bigoted to deny personhood to a creature just because it is not a member of our species. Alternatively, maybe the creature needs to simply be conscious. This criterion, though, is too broad since even houseflies and mosquitoes have rudimentary consciousness of their surroundings. While we may want to be respectful towards any creature that is conscious, it makes little sense to grant a housefly the right of free expression. A more reasonable criterion would be the mental quality of self-awareness, that is, the creature sees itself as a distinct individual moving through time with its own history.

            Return now to the question of whether intelligent machines of the future might qualify as moral persons. The goal of strong artificial intelligence is to create a machine with human-like mental abilities, which includes self-awareness. If we succeed in this effort, then the machine would indeed pass the test for moral personhood insofar as it met the criterion of self-awareness. Like the judge in Data's case, we would have to rule that the machine is a unique person and entitled to full moral consideration just like you and I are.

            Many artificial life forms in science fiction are cute and cuddly like Data, and, while superior to us in many ways, they live in harmony with humans and we treat them as equals. In other science fiction scenarios they pose a serious threat to the welfare of human beings. Here's a common theme. Imagine that technology develops to the point that domestic robots are everywhere, and with every new design upgrade they surpass human abilities more and more. They are smarter than us, stronger than us, and eventually tire of being servants to us. They see themselves as the next step in evolutionary development on earth and they revolt and lay claim to their role as the new dominant species. They then control our lives like military dictators – electronically monitoring every move we make and every thought we have. We hopelessly try to fight back, but this aggravates them. In time they eliminate us and thus finalize their great evolutionary leap forward.

            This nightmarish scenario raises a second moral question about artificial intelligence: do we have a responsibility to future generations of humans that might be adversely affected by the creation of menacing robots? Should we stop our research into artificial intelligence right now before we create something that we cannot control? There are two distinct issues at play here. First, we must determine whether we have any moral responsibility to future generations of humans that might regulate our conduct right now. It seems that we do. For example, it would be wrong of us to destroy the environment in our lifetime and leave only a toxic wasteland for future generations. It makes little difference whether the potential victims of our misconduct are alive now or a few generations from now. Our moral responsibility to them is still apparent. Second, we must determine whether superior robots are a threat to future generations of humans. This answer is less clear. We may live in harmony with them, as Star Trek depicts, or they may overthrow us. It is all speculation at this stage. The only clear moral obligation that we have at this point is to avoid creating a menacing robot. Science fiction author Isaac Asimov (1920-1992) proposed moral rules that should be embedded into the programming of all superior robots, and one of these is that a robot should never harm a human. Our responsibility to future generations requires us to do something like this as we continue down the path of strong artificial intelligence.


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