Philosophy of Mind

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The mind seems to occupy a special place in the world. It is the seat of thought and feeling, of rationality and moral concern. Is it fundamentally different from the other things we find in the natural world? Is it possible for the mind to be investigated scientifically? Can one ever really know what is going on in the mind of someone else?

Such questions delineate the subject matter of the philosophy of mind. The central problem in this area is the mind-body problem : the project of finding an account of the mind that locates it in the broader physical world. While this problem does not exhaust the philosophy of mind, one's response to it imposes substantial constraints on what one may say about other questions in this area.

One of these questions concerns mental causation. It seems obvious that what happens in the mind can bring about physical events in one's body and vice versa. If, however, the mental is radically different from the physical, such causal commerce may seem problematic. A related question concerns the prospects for psychology. If there is difficulty in supposing that causal laws govern the mental, what sorts of results can we expect from the scientific investigation of the mind?

Another question concerns epistemology. The problem of other minds is the project of explaining how one can know about the minds of others. The problem arises as a result of an asymmetry between how one knows about one's own mental states and how one knows about others' mental states. I know what I am thinking or feeling by a peculiar means devoid of inference from more basic bits of knowledge. I do not have to make observations on the basis of which I find out what is going on in my mind. Indeed, I may be unable to err about my own mental states. By contrast, I cannot know what someone else is thinking or feeling without observing their behavior, and the inferences I make are plainly subject to error. One may wonder if such inferences are ever justified. Even if they are justified, one may wonder how it is possible that the very same kind of phenomena may be known in such radically different ways.

These questions presume, of course, that we know how to sort the mental from the nonmental in the first place. Two features seem especially characteristic of the mental. First, many mental states exhibit what is known as intentionality : They have content directed at the world; they are about things. The belief that the earth is flat, for example, has as its content the proposition that the earth is flat; the fear of flying, for another example, is about flying. Second, any mental state involving an experience displays the striking feature that it makes sense to speak of what it is like to be a particular kind of creature having that kind of experience. Mental states having this what-it-is-like character may be called phenomenal states, and if someone is in such a state, we may say that that person is phenomenally conscious.

Some remaining questions in the philosophy of mind aim at more specific kinds of mental phenomena. What is the difference between an emotion and a mood? How is an intention to act related to a desire to act? Such questions often branch into other areas of philosophy: the philosophy of action, of responsibility, and so on.

The Mind-Body Problem

Leaving the notion of reduction at an intuitive level, we may distinguish the two dominant positions on the mind-body problem as follows. Materialism (or physicalism) is the thesis that that the mental reduces to the physical. More cautiously, every mental entity is ultimately nothing above and beyond the physical entities that exist. Once certain physical entities are in place, nothing extra is needed for the mental entities to exist as well. By contrast, dualism is the thesis that the mental and physical are ultimately distinct, so that neither reduces to the other. What makes the mind-body problem a problem is that we seem to have powerful evidence for both of these incompatible positions.

On the one hand, the physical workings of the brain apparently suffice to account for our behavior. If the mind is not in some way reducible to the brain, it is hard to see what room there could be for the mind to play any role in our behavior. Yet it surely does play such a role. Further, it is plain that events affecting the brain have systematic effects on the mind as well. So simplicity favors eliminating the mind as an extra entity.

On the other hand, the mind resists such a reduction. It is hard to see how the physical aspects of anything could add up to its having thoughts or feelings. Any putative creature with a mind may well be a mindless automaton. The point is made most vivid if we consider a physical organism built out of the very same physical ingredients as you, the reader. It seems possible that such an organism may yet be mindless, despite its physical and behavioral similarity to a creature that has a mind. If this is right, then clearly what makes you a creature with a mind does not automatically accompany your physical characteristics; it is something over and above the physical. Hence, materialism is false.

Materialism and dualism are not the only options. One other option is idealism (or phenomenalism), according to which it is the physical that ultimately reduces to the mental. Further, there is the view that neither the mental nor the physical reduces to the other. Rather, both reduce to some third, neutral entity. This position is sometimes known as neutral monism. Neither alternative has been widely endorsed. Idealism is apt to seem simply incredible, and neutral monism may seem frustratingly mysterious.


René Descartes developed the most famous form of dualism, a form known as Cartesian substance dualism. On this view, the mind is a substance (and hence capable of existing on its own without any distinct supporting entities, such as a body). It is distinct from any physical object in that it is essentially without spatial extension. A dualist need not adopt all of these tenets, however. A dualist need hold only that mental propertiessuch as being in pain or intending to leave the roomare in principle independent of any combination of physical properties.

As noted above, there is considerable pressure to opt for materialism in order to make sense of the causal role that the mental has in affecting our behavior. One route that the dualist might take in the face of this pressure is to endorse epiphenomenalismthe view that, contrary to appearances, mental events never cause physical events. There would then be no need to accommodate the causal role of the mind. This advantage is offset, however, by a grave difficulty in accounting for one's knowledge of other minds. When I know what someone else is thinking, my primary evidence is that person's behavior. In treating this as evidence, I presume that part of what brings about such behavior is the person's mental states. Epiphenomenalism undercuts this presumption and throws into doubt the value of such evidence.

There is no requirement that a dualist be an epiphenomenalist. Perhaps the most attractive form of dualism is one that maintains causal interaction between mind and body while rejecting the supposition that the mind is a substance. The view known as emergentism does exactly this. (For a classic defense of the view, see C. D. Broad's The Mind and Its Place in Nature.) Emergentism may be characterized by three theses. First, it rejects the view that the mind is a substance, maintaining only that there are two types of properties, mental properties and physical properties (property dualism). Second, it claims that once an organism reaches a certain level of complexity, the laws of nature dictate that it will then have various irreducible mental properties. Third, it holds that these mental properties subsequently make a difference to the organism's behavior. More precisely, in the presence of mental properties, the physical elements of the organism behave differently from what one would expect on the basis of just the general laws governing those physical elements when not assembled in this special fashion. The emergentist thus makes the bold empirical claim that we have in effect only an incomplete view of the laws of physics, that if one were to examine the physical events occurring in creatures with minds, one would find that the usual laws do not apply.

Logical Behaviorism

In the first half of the twentieth century, logical behaviorism held sway as the main alternative to dualism. On this view, any statement about the mental can be translated into a statement about behavioral dispositions. A statement such as "Amy is in pain" is synonymous with some such statement as "Amy is disposed to wince, cry out, etc." Since wincing and crying out are themselves physical events, it seems that something purely physical can be disposed to undergo such events. If being in pain is merely being thus disposed, then a purely physical thing can be in pain. (A seminal statement of this view may be found in Gilbert Ryle's The Concept of Mind.)

Logical behaviorism has several attractive aspects. First, it makes good sense of our knowledge of other minds. If pain is simply the disposition to wince, cry out, or the like, then I can know that someone is in pain simply by observing those behaviors. Second, it fits happily with the familiar picture of how we come to learn psychological terms, specifically, that one learns what others mean by the word "pain" by observing that pain is attributed to people on the basis of their overt behavior. Third, the view explains why we have apparent a priori knowledge of the links between certain mental states and certain behaviors. We do not have to gather empirical evidence to support the claim that wincing is typically a sign of pain.

Logical behaviorism nonetheless faces a very basic problem, namely, that no proposed translation is in fact plausible unless it makes use of further mental terms. Consider again the statement "Amy is in pain." It seems possible for this to be true even when she is not disposed to wince, cry out, or the like. Suppose, for instance, that she wishes not to let anyone discover her pain and is thus determined to suppress any overt indications of it. She will then not be inclined to behave in those ways.

Of course, we could try to understand the behavioral disposition in a more complex fashion. We might unpack the disposition claim as follows: "Amy is in such a state that if she were to feel uninhibited, she would wince, cry out, etc." The situation in which she never displays such behaviors because she is determined to suppress such signs is no counterexample to this translation. But if this is the translation on offer, then we have not succeeded in showing how a purely physical entity could be in pain, since the complex characteristic assigned to Amy is already mental in part in that it refers to feeling uninhibiteda mental characteristic in its own right. In general, any attempt to characterize a behavioral disposition seems bound to include such a reference.

The Identity Theory

The identity theory rose to prominence in the middle of the twentieth century, succeeding logical behaviorism as the leading materialist theory. The view is simple: Every type of mental state is identical with some type of physical state, probably a neurophysiological state. (An extensive overview of the sorts of considerations that helped lead many philosophers to the identity theory can be found in Herbert Feigl's extensive essay "The 'Mental' and the 'Physical'.")

What is novel in the identity theory is not so much its simple positive claim as its disavowals: The identity claim is not accompanied by any claim about translation. This feature of the identity theory enables it to avoid many of the usual objections to materialism. This virtue can be illustrated by working with a well-known example. Consider the claim that being in pain is identical with some type of brain process, say, having one's C-fibers firing. Identity theorists suppose that the relation between "being in pain" and "having one's C-fibers firing" is analogous to the relation between "the morning star" and "the evening star": the terms have different senses, but the same referent. Other favored examples include the identity of lightning with a kind of electrical discharge, or the identity of heat with molecular motion. In each case, the identity can only be discovered empirically; it cannot be discovered by a priori analysis of the meanings of the terms.

While the a posteriori character of these identity claims is a key appeal of the identity theory, it is also the source of important objections. One such objection was made famous as "objection 3" in J. J. C. Smart's classic paper "Sensations and Brain Processes" (1959). If an identity statement of the form "M = P " is a posteriori, different concepts must be associated with "M " and "P." Those different concepts involve different properties that pin down the referent of "M " and "P." For example, with the identity "the morning star = the evening star," the first name is associated with certain properties, such as being visible in the morning, that are not associated with the second name. Now turn to the alleged identity of pain with C-fiber firing. By analogy, we should conclude that despite the truth of this identity, there are nonetheless two distinct sets of properties, those associated with "pain" and those associated with "C-fiber firing." The objection, finally, is that the property associated with "pain" is a mental property that has yet to be identified with anything physical. Any a posteriori identity between the mental and physical will leave an unreduced residue of mental properties, and these mental properties undermine materialism.

Smart's response to this objection was to acknowledge that there must be different senses associated with the mental and physical terms of the identity while insisting that the sense of the mental term can be explained without appeal to any further mental properties. For instance, he claims that the sentence "I see a yellowish-orange afterimage" is equivalent in meaning to "There is something going on in me which is like what goes on in me when I see an actual orange in good light." The vocabulary in this second statement is topic-neutral in the sense that it is silent on the nature of what is going on; it may or may not be a physical process. When we identify the experience of a yellowish-orange afterimage with a type of brain process, that identity is justified by the empirical evidence that shows that the named type of brain process is in fact what is going on when one sees an actual orange in good light.

It is worth stressing here that, while the identity theorists advertised a lack of commitment to translations of psychological sentences, this sort of objection seems to force them to providing translations nonetheless. Their translations might prove to be just as dubious as the behaviorist's translations.

Anomalous Monism

One important challenge to the identity theory is posed by anomalous monism, the view championed by Donald Davidson and made famous in his essay "Mental Events." The view may be defined as a combination of one positive thesis and one negative thesis. Positively, it holds that each particular mental event is also a particular physical event, though categories of mental events cannot be equated with categories of physical events. Anomalous monism thus endorses a thesis of token identity, but not type identity. The negative thesis is that the mental is anomalous: there are no strict laws involving mental events as such. This anomalism allegedly blocks the discovery of laws relating the mental and the physical, laws apparently needed to justify a claim of identity between mental and physical properties.

The negative claim is aimed directly at the identity theory; it seeks to undercut potential sources of empirical support for that view. It is worth noting that even if anomalism is consistent with the identity theory, it is certainly significant for psychology, since it rules out the ambition of psychology to uncover strict laws governing the mental.

The positive thesis also challenges the identity theory, albeit indirectly, in that it suggests that one can be a materialist without being an identity theorist. If it suffices for materialism to say that each particular mental event is identical with some physical event, then a materialist may rest content with such instead of holding out for the more ambitious theory of type identity. Yet few philosophers are convinced that a thesis of token identity is sufficient for materialism. Intuitively, a materialist must hold, at a minimum, that how someone is mentally depends on how that person is physically. The thesis of token-event identity does not secure this result.


The idea that how things are physically must determine how things are mentally may be captured by the notion of supervenience, also introduced into the philosophy of mind by Davidson. To say that mental properties supervene on physical properties is (roughly) to say that any two creatures that are exactly alike physically must also be exactly alike psychologically. There may be no neat match-up of mental and physical properties, but supervenience implies that how things are mentally is fixed by how things are physically.

The notion of supervenience is in this way useful for formalizing a kind of dependence of the mental on the physical, although there have been many subtly different ways of making the notion precise. There is an important limitation to any supervenience thesis, however, in that the thesis itself leaves unanswered questions as to why and how the mental is determined in this fashion. To answer these questions, it seems that a more committed theory of the nature of the mental is needed.


A distinct challenge to the identity theory came in the form of functionalism. This is the view that mental properties are functional properties, that is, properties defined by the causal or functional roles they play. Consider the property of being a laundry detergent. Something is a laundry detergent if and only if it can combine with water in a washing machine to clean clothes. Various different chemicals can play this role equally well. When a particular chemical plays this role, it is said to realize the property of being a detergent. Since many different chemicals can play this role, the property of being a detergent is multiply realized and cannot be identified with any one of its realizers. (A seminal paper advocating functionalism is Hilary Putnam's "The Nature of Mental States.")

One motivation for functionalism is the conviction that it is in fact very unlikely that there is a single physical property to be found in all creatures sharing a given mental state. Functionalism accommodates this conviction by allowing mental properties to be multiply realized, and it does so without giving up on materialism, as an individual can have a functional property solely in virtue of his physical characteristics.

Functionalism has also been found attractive because of the apparent similarity between minds and computers. Consider what it is for a computer to run a program. The same program can be run by many different sorts of machines, so long as they have distinguishable states that play the right roles relating inputs, outputs, and each other. If the mind is akin to a computer, mental states may plausibly be classified as functional, relating sensory inputs, behavioral outputs, and different internal states. (For an important challenge to this analogy, see John Searle's "Minds, Brains, and Programs.")

A further appeal of functionalism is that it promises a degree of autonomy for psychology. If mental properties are multiply realized, then one can investigate what mental properties do without worrying about the specific physical characteristics of the underlying realizers. It is, of course, controversial how much autonomy this provides.

Even if we opt for functionalism, there remains much work to be done by way of locating the right sorts of functional properties to identify with various mental properties. The two distinctive features of the mind mentioned earlierintentionality and phenomenal consciousnessprovide targets for such work.


Theories of intentionality have generally taken either of two forms. They differ primarily in whether they determine the content of a mental state by appeal to the overall functioning of the mind in question or by appeal to individual mental states in isolation. On the former (interpretational) approach, a subject S has the belief that P just in case the belief that P appears in the overall assignment of intentional states providing the best interpretation of S. The details of the theory depend on what it takes to amount to a good interpretation. Typically, the idea is that the theory must predict the behavior of S and make S 's thoughts and actions by and large rational for someone in that environment.

The other (causal/informational) approach, which focuses on specific connections between particular brain states and states in the world, is encouraged by the idea that we may be able to distinguish within states such as believing that P and hoping that P a common elementa representation that P for which an independent physicalist theory can be given. The physical state N may represent that P by virtue of a causal link, in that someone in state N has the information that P. On a very simple version of this view, N represents that P if and only if the only thing that can cause someone to be in N is the fact that P. This simple version fails to make room for false representations, however; some way is needed to distinguish those causes of the representing state that fix its content and those that do not. (Seminal works in this area include Fred Dretske's Knowledge and the Flow of Information and Jerry Fodor's Psychosemantics. A useful survey may be found in the anthology Mental Representation: A Reader, edited by Stich and Warfield.)

Content Externalism

Whatever theory of content one develops, an important constraint is imposed by content externalism. This is the view that the content of someone's mental states is determined not solely by that person's intrinsic features; the larger social and historical environment in which that person is embedded makes a difference. An easy route to seeing the point is to consider beliefs about particular individuals. Suppose that Amy and Basil are friends, that Amy believes that Basil is intelligent, and further, that Basil has a twin about whom Amy knows nothing. Amy's belief is plainly about Basil, not his twin. Yet if the situation were reversed, so that Amy was acquainted with Basil's twin instead of Basil, her belief would have had a different content, even though she would have been intrinsically the same in both cases. Hence, the contents of one's mental states may vary while one's intrinsic features remain unchanged. (Two fundamental papers about content externalism are Hilary Putnam's "The Meaning of 'Meaning' " and Tyler Burge's "Individualism and the Mental.")

This observation has raised two concerns. First, some worry that externalism is problematic for the view that intentional mental states can play a causal role in determining behavior. The worry, crudely put, is that since content is determined by wider environmental factors, content can play a causal role in behavior only if those wider environmental factors themselves play a causal role, which seems mysterious. A second concern is that externalism may be incompatible with the privileged access to our own minds that we seem to have. We need not investigate our environment to know what we think; yet if the contents of our thoughts depend on that environment, it may seem mysterious how we manage such a feat. These two problems have motivated some philosophers to introduce a notion of narrow contentmental content determined solely by the intrinsic features of the agent. If there is such a thing as narrow content, any theory of intentionality needs to accommodate it as well as content individuated in a more ordinary fashion.

Phenomenal Consciousness

The second distinctive aspect of the mind with which materialists must contend is phenomenal consciousness. What sort of physical and/or functional property can ensure that its bearer is undergoing an experience?

Many positive approaches to phenomenal consciousness take their cue from the fact that phenomenal states seem bound up with intentionality. Consider, for instance, what it is like to look at a bright red tomato. That experience plausibly represents the world as being a certain way: as containing a bright red tomato. One may even argue that all phenomenal states include such content. The state of pain, for instance, may represent one's body as being damaged.

What makes a state phenomenal, however, is not simply its having a certain content. Something else must be added to distinguish the mere belief that there is a bright red tomato in front of one from the visual experience of a bright red tomato in front of one. A variety of proposals have been offered as to what might make the difference. On one option, the content of a phenomenal state plays a rather different functional role in the overall system than the content that attaches to a mere belief. On another, a phenomenal state is a representational state that itself is represented by some other, higher-order representational state.

Whatever the merits of these theories, few would hold that they can be seen to be true simply as a matter of conceptual analysis. It is simply too easy to imagine situations in which the proposed physical and functional conditions are met even while nothing is experienced at all. Indeed, it seems quite conceivable that a being could have all the various physical and functional properties that we ourselves have and yet be devoid of phenomenal consciousness. Such creatures are known as philosophical zombiesphysical duplicates of ourselves for whom all is "dark inside." (For influential discussions, see David Chalmers, The Conscious Mind ; Peter Ludlow et al., eds., There's Something about Mary ; and Ned Block et al., eds., The Nature of Consciousness.)

The fact that we can easily conceive of such zombies does not, of course, settle the issue in favor of dualism. As is familiar from the work of the identity theorists, the identity in question may be a posteriori. Consider the case of heat and molecular motion again. There is, in fact, no possible situation in which heat is present without molecular motion; nonetheless, we can apparently conceive of such a situation. We may explain away that apparent conceivability, however, by pointing out that we could then be imagining a world in which something other than heat appears to be heat, because, we imagine, this other thing produces heat sensations. We have misdescribed the genuine possibility we imagined.

The materialist appears to be obligated to offer a similar sort of story explaining away our apparent ability to conceive of zombies. There is, however, an important difference between the psychophysical case and the case of phenomenal states. In the heat example, we could distinguish between the appearance of heat and the heat itself, but in the case of phenomenal states, it is unclear that a comparable distinction can be drawn. (This well-known argument is found in Saul Kripke's Naming and Necessity.)

The difficulty here is related to one discussed earliernamely, that made famous as "objection 3" in Smart's classic defense of the identity theory. There the worry turned on the implications of saying that mental and physical terms are associated with quite distinct concepts. The materialist needs to offer some story about those concepts that allows us to explain the a posteriori character of the identity claim, and the apparent possibility of zombies, in a way consistent with the claim that all properties are ultimately nothing over and above physical properties. Whether any such story is available remains an extremely controversial question.

See also Behaviorism; Dualism in the Philosophy of Mind; Functionalism; Mind-Body Problem; Physicalism; Reductionism in the Philosophy of Mind.


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D. Gene Witmer (2005)

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