The distinction between the mental and the physical is central both to commonsense thinking about the world and to many philosophical, scientific, and religious theories. Perhaps it is as important to human thought as the distinction between fact and value, and between the empirical and the a priori. This entry will focus both on the role of the distinction in analytic philosophy and on various proposals about how it is to be understood.
The mental/physical distinction plays a role in two main areas of philosophy. First, in philosophy of mind, many arguments and issues are formulated in terms of it. Philosophers who advance physicalist theories about the mind argue that phenomenal consciousness (for example) is a physical phenomenon similar in kind to electricity or sexual reproduction; dualists deny this, saying that what we have here are two fundamentally different sorts of thing or two different characteristics of things. Second, in the philosophy of science and related parts of metaphysics, there is the issue of how to formulate the picture of the world that is presented to us by modern science. Many contemporary philosophers assume that this picture is in essence a physicalist one, and mean by this that the world-\view implicit in modern science bears important affinities with the materialism (also known as physicalism) of the seventeenth and eighteenth centuries, in particular that of La Mettrie and Hobbes. A natural assumption is that to properly evaluate whether the worldview of modern science really is a kind of physicalism, and to fully understand the related dispute in philosophy of mind between physicalism and dualism, one would need to clarify the mental/physical distinction. So what exactly is it?
There seems to a tacit general understanding of the mental/physical distinction but no rigorous idea of how it is to be drawn exactly —the implicit understanding has not been made explicit. That we understand the distinction in some sense is indicated by the fact that we spontaneously sort various features or characteristics of people or animals into two lists, the mental and the physical. So, to focus on a particular person Jones, we have on the mental side the fact that he knows where his car keys are, has itchy feet, wants tickets to the opera, and so on. On the physical side, we have the fact that he weighs 170 pounds, is currently located in Detroit, Michigan, is moving in such and such a direction with such and such a speed, and so on. The problem comes when we try to say in any detail what the occurrences of "and so on" mean. What precisely places a feature in the mental list, and what distinguishes those on the mental list from those on the physical? What groups weighing 170 pounds together with being currently located in Detroit, Michigan, and sets it apart from having itchy feet ? Or take some other property of Jones not mentioned so far: for example, that his brain is releasing certain hormones into his bloodstream—is it mental or physical? If, as it seems natural to say, it is physical, what makes it so?
There is no shortage of proposals in the literature about what makes it so, and more generally about how to understand the mental/physical distinction, but all of them face problems, and none commands widespread assent. What immediately follows is a brief catalogue. The first, and historically the most important, proposal is that of Descartes (1641). Descartes said that being physical (or material) is just being extended in space; likewise, he said, the essence of the mind is to think, to engage in the activity of thinking. Descartes went on to argue that, if this is the way to draw the mental/physical distinction, dualism in philosophy of mind is true. This clarification of the distinction is straightforward, but it also has a number of drawbacks. First, we think of matter as something that occupies space, rather than being identical to space—but Descartes notoriously makes no room for such a distinction. Second, there are intuitively physical forces—such as the force of gravity—that would not be classified as physical from Descartes' point of view. Third, the idea that the essence of the mind is to think apparently excludes mental states that are sensory rather than cognitive and those that do not involve some sort of mental activity.
The second proposal—one might view it as an updated version of Descartes—draws the mental/physical distinction by appealing to two ways in which we find out about the world: introspection and perception. On this view, something is mental just in case we can find out about it, at least in principle, by introspection, whereas something is physical just in case we can find out about it, at least in principle, by perception. But this proposal faces difficulties also. One problem is that many things that seem intuitively physical are not directly available to perception even in principle—for example, subatomic particles. One might weaken the criterion and say that something is physical just in case we can find out about it either by perception or by inference from perception. But the problem now is that the mental states of other people are such that we can find out about them by inference from perception; hence the weakened account entails the physicality of those mental states. Another problem with this second proposal is that it is not clear what the category of introspection is. Introspection seems to be the faculty by which we find out about our own mental goings-on—but this drains the idea of content.
The third proposal, prominent in the work of Thomas Nagel (1974, 1986), explains the mental/physical distinction as a special case of the contrast between the subjective and the objective. One obvious problem here is that the distinction between the subjective and objective is itself unclear; it is no advance to take subjective to mean "mental." But Nagel himself interprets the distinction as concerning different conditions of understanding: An objective truth or fact is one that can be understood from more than one point of view, whereas a subjective truth or fact is one that can be understood from at most one point of view. One objection to this is that there are psychological phenomena that are objective in Nagel's sense; presumably, the psychological properties attributed to humans by theoretical as opposed to folk psychology are as objective as any anything else. (These properties are not available to introspection either—and this causes a problem for the previous proposal, too.) A second objection is that the distinction between mental and physical is now a distinction within the realm of things that can be understood. But it is quite unclear that something is physical only if it is understandable.
The two proposals we have just considered inherit from Descartes the idea that we need criteria both for the mental and the physical. But contemporary philosophers have also explored the more cautious idea that one might define directly what it is for something to be physical, leaving aside the question of what it is for something to be mental. Hence, the fourth proposal is that something is physical just in case it is the sort of thing that physical theory tells us about or perhaps is entailed by the sort of thing physical theory tells us about. The basic objection to this view is Hempel's dilemma (Hempel 1969; see also Crane and Mellor 1990). Hempel's dilemma is that if the physical theory in question is contemporary physics, this proposal entails that physicalism is obviously false—after all, nobody believes that contemporary physics is complete; on the other hand, if the physical theory in question is some idealized or future physics, then the proposal entails that physicalism is empty—after all, who knows what some idealized or future physics will include? Some (for example, Smart 1974) respond by asserting that it is rational to believe that contemporary physics is complete. Although there is something right about this—surely it is rational to believe contemporary physics—the implicit suggestion that we should define the physical in terms of contemporary physics is implausible. Medieval impetus physics (for example) is a false and outmoded theory, but the property that objects have according to it—namely, impetus—is a physical property nonetheless.
According to a fifth proposal—sometimes called the paradigm physical object view—something is physical just in case it is the sort of thing required by or entailed by a complete account of the intrinsic nature of paradigmatic physical objects and their constituents (Block 1980; see also Feigl 1967). The basic idea of this view is that we have some paradigms of physical objects—trees, stones, planets, toasters—and that the physical is whatever you need to explain them. One problem with this view is that it is circular—it explains the physical in terms of physical objects. (The same problem afflicts the previous proposal, which defines the physical in terms of physical theories.). Another problem for this view is that if physical objects turned out very different from how they appear—if, for example, they had a spiritual essence—physicalism and idealism would on this view be indistinguishable.
Perhaps it is unsurprising on reflection that the proposals just reviewed run into difficulties; they are all attempts at saying something positive about what the physical consists in. The sixth proposal is the negative one of saying that physical just means "nonmental" (for example, Levine 2001). One problem with this idea is that it assumes some criterion or mark of what it is to be mental; for example, that something is mental just in case it has phenomenal character or intentionality or both. And someone might question or reject both proposals either singly or in combination. But the more serious problem for the via negativa is that, construed as a definition of the physical, it gets things quite wrong. A vitalist, for example, thinks that living things instantiate properties—élan vital —which are both nonmental and nonphysical. However, while vitalism might be as false and outmoded as medieval impetus physics, it is not self-contradictory.
In view of the fact that every extant proposal about how to clarify the mental/physical distinction faces problems, it is natural to wonder whether there is any clear distinction here at all. Perhaps this is a distinction that we draw in ordinary thought but is something that should be done away with in serious scientific or philosophical descriptions of the world. That is the proposal that a number of people have found themselves drawn to, including Chomsky (2000).
One response to this sort of scepticism is that it is driven by overly high standards of clarity. True, it is hard to clarify the mental/physical distinction, but this difficulty does not mean that there is no such distinction—for the same thing might be said for many interesting distinctions and concepts. A different (but consistent) response asks us to look again at why we wanted a clarification of the mental/physical distinction in the first place. If the answer is intellectual curiosity, the Chomksian view is as reasonable as any other. But Chomskian skepticism gains much of its power from the further idea that various intellectual projects in philosophy of mind and science make no sense unless the mental/physical distinction can be clarified. But in fact it is not clear that this is so. Earlier we noted that various projects in philosophy of mind and science are formulated in terms of the mental/physical distinction. But it does not follow that the distinction is essential to these projects. If the mental/physical distinction can be shown to play only an illustrative or inessential role in these projects, then skepticism about the distinction itself—whether or not it is warranted—will not be as consequential as it would otherwise appear to be.
See also Chomsky, Noam; Descartes, René; Dualism in the Philosophy of Mind; Hempel, Carl Gustav; Hobbes, Thomas; Idealism; La Mettrie, Julien Offray de; Nagel, Thomas; Philosophy of Science, History of; Philosophy of Science, Problems of; Physicalism.
Block, N. "Troubles with Functionalism." In Readings in the Philosophy of Psychology, vol. 1, edited by N. Block. Cambridge, MA: Harvard University Press, 1980.
Chomsky, Noam. New Horizons in the Study of Language and Mind. Cambridge, U.K.: Cambridge University Press, 2000.
Crane, T., and D. H. Mellor. "There Is No Question of Physicalism." Mind 99 (1990): 185.
Descartes, Rene. "Meditations on First Philosophy." In The Philosophical Writings of Rene Descartes, edited by John Cottingham. Cambridge, U.K.: Cambridge University Press, 1985. First published in 1641.
Feigl, H. The "Mental" and the "Physical." Minneapolis: University of Minnesota Press, 1967. First published 1958.
Nagel, T. The View from Nowhere. New York: Oxford, 1986.
Nagel, T. "What Is It Like to Be a Bat." Philosophical Review 4 (1974): 435–450.
Smart, J. J. C., "The Content of Physicalism." Philosophical Quarterly 28 (1974): 239—241.
Daniel Stoljar (2005)