Perhaps the problem that most people think of first when they think about the nature of the mind is whether the mind can survive the death of the body. The possibility that it can is, of course, central to many religious doctrines, and it played an explicit role in Descartes’s formulations of mind-body dualism, the view that mind and body constitute fundamentally different substances (see below Substance dualism and property dualism). However, it would be a serious mistake to think that contemporary controversies about the nature of the mind really turn on this remote possibility. Although it can often seem as though debates between dualism and reductionism—the view that, in some sense, all mental phenomena are “nothing but” physical phenomena—are about the existence of disembodied spirits, virtually none of the contemporary forms of these disputes take this possibility seriously, and for good reason: there is simply no serious evidence that anyone’s mind has ever survived the complete dissolution of his body. Purported “out of body” experiences, as well as people’s alleged memories of events occurring minutes after they are pronounced dead, are no more evidence of disembodiment than are the dreams that many people have of witnessing themselves doing various things.

There is, however, an interesting problem related to the question of disembodied souls, one that can be raised even for someone who does not believe in that possibility: the problem of personal identity. What makes someone the same person over time? Is it the persistence of the same body? Suppose that the cells in one’s body became diseased and that it was medically possible to replace them one-by-one with new cells. Arguably, if the replacement was extensive enough, one would be the same person with a new body. And it is presumably something like this possibility that people envision when they imagine reincarnation.

But if it is not the body that is essential to being the same person, then it must be something more purely psychological—perhaps one’s memories and character traits. But these come and go over a lifetime; most people remember very little of their early childhoods, and some people have trouble remembering what they did only a week earlier. Certainly, many of one’s interests and character traits change as one matures from childhood to adolescence and then to early, middle, and late adulthood. So what stays the same? In particular, what is it that underlies the peculiar concern and attachment one feels about the even distant future and past portions of one’s life? It is not at all easy to say. (Note that this is a problem as much for the believer in immaterial souls as for the person who believes only in bodies. To vary the story by Mark Twain, how would it be possible, without some criterion of personal identity, to distinguish the case of a prince being reincarnated as a pauper from the simple case of a pauper being born shortly after the death of a prince?)

The identity theory

The simplest proposal for explaining how the mental is nothing but the physical is the identity theory. In his classic paper “Materialism” (1963), the Australian philosopher J.J.C. Smart proposed that every mental state is identical to a physical state in the same way, for example, that episodes of lightning are identical to episodes of electrical discharge. The primary argument for this view is that it enables a kind of economy in one’s account of the different kinds of things in the world, as well as a unification of causal claims: mental events enter into causal relations with physical ones because in the end they are physical events themselves. This view is also called reductionism, which unfortunately conveys the misleading suggestion that the mental is somehow “made less” by being physical. This is of course mistaken, as lightning is no less lightning for being reduced to electrical discharge, and water is no less water for being reduced to H₂O.

The comparisons with lightning and water, however, carry what many philosophers have thought to be an implausible implication. Although every instance of lightning is an instance of the same type of physical state—electrical discharge—it is doubtful that every instance of believing that grass grows, for example, is also an instance of the same type of physical state—i.e., the excitation of specific neurons in the brain. This is because it seems possible for two people to have brains composed of slightly different substances and yet to share the same belief or other mental state. Likewise, it could be that there are extraterrestrials who believe that grass grows, though their brains are composed of materials very different from those that make up human brains. Why should reductionists rule out this possibility?

This unwanted implication can be avoided by noticing an ambiguity in identity statements between types and tokens. According to a “type-identity” theory, every type of mental phenomenon is some (naturally specifiable) type of physical phenomenon. This is quite a strong claim, akin to saying that every letter of the alphabet is identical to a certain type of physical shape (or sound). But this seems clearly wrong: there is quite a diversity of shapes (and sounds) that can count as a token of the letter a. A more reasonable claim would be that every token of the letter a is identical to a token of some type of physical shape (or sound). Accordingly, many materialist philosophers have retreated to a “token-identity” theory, according to which every mental phenomenon is identical to some physical phenomenon.

The distinction between types and tokens of mental phenomena may afford a way for the reductive physicalist to concede a point to the traditional dualist without giving up anything important. This is because distinguishing between types of phenomena can be regarded as a way of distinguishing between different ways of classifying them, and there may be any number of ways of classifying a given phenomenon that are not reducible to each other. For example, every piece of luggage is presumably a physical object, but no one believes that “luggage” is a classification that can be expressed in—or reduced to—physics, and no one has ever seriously proposed a “luggage-physics” dualism. If this is the kind of dualism that the mental involves, it would therefore seem to be quite innocuous.

Even if the identity theory is restricted to token-identity claims, however, there are still problems. One simple example concerns the relation of many mental phenomena to physical space. As noted earlier (see above Abstract and concrete), it is ordinarily quite unclear exactly where such things as beliefs and desires are located. They are often said to be “in the head”—but where in the head, exactly? Or, to take a harder example, mental images seem to have certain physical properties, such as being oval and vividly coloured. But if such images are to be identified with physical things, then it would seem to follow that those things should have the same physical properties—there should be oval, vividly coloured objects in the brains of people who experience such images. But this is absurd. So it would seem that a mental image cannot be a physical thing. (Arguments of this sort are sometimes called “Leibniz-law arguments,” after a metaphysical principle formulated by the German philosopher Gottfried Wilhelm Leibniz [1646–1716]: if x = y, then whatever is true of x must also be true of y). Other, more technical problems with the identity theory (pressed most vigorously by the American philosopher Saul Kripke) are beyond the scope of this article. The cumulative effect of these difficulties has been to make philosophers wary of couching reductionism in terms of identity.

An alternative is to say not that mental phenomena are identical to physical phenomena but rather that they are “constituted” by them. Consider a porcelain vase. Suppose someone were to break the vase and make a statue out of all of the pieces. If both the vase and the statue are identical to the pieces, it would follow that the statue is identical to the vase, which is absurd. So the vase and the statue are not identical to the pieces but merely constituted by (or composed of) them.

Physicalists think that it is possible to say more than this. Not only is every mental phenomenon constituted by physical phenomena, but every property of the mental crucially depends upon some physical property. Physicalists think that mental properties “supervene” on the physical, in the sense that every change or difference in a mental property depends upon some change or difference in a physical property. It follows that it is impossible for there to be two universes that are physically identical throughout their entire history but that differ with respect to whether a certain individual is in pain at a particular time.