Physicalism is the doctrine that everything that exists in the spacetime world is a physical thing, and that every property of a physical thing is either a physical property or a property that is related in some intimate way to its physical nature. Stated this way, the doctrine is an ontological claim, but it has important epistemological and methodological corollaries.

Physicalists in general will accept the following thesis of "ontological physicalism" (Hellman and Thompson 1975): Every object in spacetime is wholly material -- that is, it is either a basic particle of matter (proton, electron, quark, or whatever) or an aggregate structure composed exclusively of such particles. Ontological physicalism, therefore, denies the existence of things like Cartesian souls, supernatural divinities, "entelechies," "vital forces," and the like. Physicalists, however, differ widely when it comes to the question of properties of physical objects -- whether complex physical systems can have properties that are in some sense nonphysical. But what is a physical property?

It is difficult to give a clear-cut answer to this question. In a narrow sense, physical properties are those properties, relations, quantities, and magnitudes that figure in physics, such as mass, energy, shape, volume, entropy, temperature, spatiotemporal position and distance, and the like. Most will also include chemical properties like valence, inflammability, and acidity, although these are not among the basic physical properties -- properties that figure in basic physical laws (in this sense entropy and temperature are not basic either). In discussions of the status of cognitive/psychological properties, physical properties are usually also taken to include such higher-level properties as biological properties and computational properties. This broad sense of physical property seems appropriate to the discussion of the question how psychological properties are related to physical properties -- that is, the MIND-BODY PROBLEM. In its broad sense, therefore, "physical" essentially amounts to "nonpsychological." This leaves our previous question unanswered: what is a physical property? Mass, charge, energy, and the like are of course important properties in current physics, but the physics of the future may invoke properties quite different from those in today's physics. How would we recognize them as physical properties rather than properties of another sort? That is, how would we know that future physics is physics?

As noted, physicalists differ on the status of higher-level properties in relation to lower-level, basic physical properties. Reductive physicalism claims that higher-level properties, including psychological properties, are reducible to, and hence turn out to be, physical properties. Opposed to reductive physicalism is nonreductive physicalism, also called property dualism, which takes at least some higher-level properties, in particular cognitive/psychological properties, to form an irreducible autonomous domain. This would mean that psychology is a special science whose object is to investigate the causal/nomological connections involving these irreducible psychological properties and generate distinctively psychological explanations in terms of them. In this view, these laws and explanations cannot be formulated in purely physical terms -- not even in an ideally complete physical theory -- and a purely physical description of the world, however physically complete it may be, would leave out something important about the world. Nonreductive physicalism, therefore, leads to the doctrine of the AUTONOMY OF PSYCHOLOGY and, more generally, the autonomy of all special sciences in relation to basic physics (Davidson 1970; Fodor 1974).

The mind-brain identity theory (Feigl 1958; Smart 1959; Armstrong 1968) is a form of reductive physicalism. This approach proposes to identify psychological properties with their neural correlates; for example, pain is to be identified with its neural substrate ("C-fiber stimulation," according to armchair philosophical neurophysiology). These mental-neural identities are claimed to be just like the familiar identities discovered by science, for example, "Water = H2O," "Light = electromagnetic radiation," and "Genes = DNA molecules." Just as the "true nature" of water is being composed of H2O molecules, advances in neurophysiology will reveal to us the true nature of each type of mental state by identifying it with a specific kind of brain state.

EMERGENTISM, a doctrine popular in the first half of the twentieth century, is a form of nonreductive physicalism (Morgan 1923; Sperry 1969; McLaughlin 1992). Its central tenet is the claim that certain higher-level properties, in particular consciousness and intentionality, are emergent in the sense that, although they appear only when a propitious set of physical conditions are present, they are genuinely novel properties that are neither explainable nor predictable in terms of their underlying physical conditions. Moreover, these emergent properties bring into the world their own distinctive causal powers, thereby enriching the causal structure of the world. FUNCTIONALISM is also often thought to be a form of nonreductive physicalism. According to this position, psychological properties are not physical or neural properties, but rather functional kinds, where a functional kind is a property defined in terms of causal inputs and outputs. To give a familiar example, pain is said to be a functional kind in that being in pain is to be in some physical/biological state that is typically caused by certain types of physical inputs (e.g., tissue damage) and that causes certain behavioral outputs (e.g., groaning, wincing, escape behavior). It is then noted that a psychological kind when given a functional interpretation of this kind has multiple physical realizers (Putnam 1967; Block and Fodor 1972; Fodor 1974); that is, the neural mechanism that realizes or implements pain in humans is probably vastly different from the pain mechanisms in reptiles, mollusks, and perhaps certain complex electromechanical systems. This is "the multiple realization argument" against reductionism: because pain is multiply realized in diverse physical/biological mechanisms, it cannot be identified with any single physical or biological property. This has led to the view that cognitive/psychological properties are at a higher level of abstraction and formality than the physical/biological properties that implement them (Kim 1992).

However, nonreductive physicalists, insofar as they are physicalists, will acknowledge that psychological properties, although physically irreducible, are in some sense dependent on, or determined by, physical properties -- unless, that is, one is prepared to take their physical irreducibility as proof of their unreality and adopt eliminativism/irrealism (or ELIMINATIVE MATERIALISM) about the mental (Churchland 1981). That is, physicalists who accept the reality of the mental will accept the mind-body SUPERVENIENCE thesis (Hellman and Thompson 1975; Horgan 1982; Kim 1984): the psychological character of an organism or system is entirely fixed by its total physical nature. From this it follows that any two systems with a relevantly similar physical structure will exhibit an identical or similar psychological character. Even emergentists will grant that when identical physical conditions are replicated, the same mental phenomenon will emerge, or fail to emerge. Supervenience is also a commitment of functionalism: systems in identical physical conditions presumably have the same causal powers and so will instantiate the same functional properties. It is a basic commitment of all forms of physicalism that the world is the way it is because the physical facts of the world are the way they are. That is, physical facts fix all the facts.

Among the facts of this world are causal facts, including those involving mental and other higher properties. The supervenience thesis implies then that these higher-level causal facts are fixed by lower-level physical facts, presumably facts about physical causal relations. The same goes for higher-level laws: under supervenience, these laws are fixed once basic physical facts, in particular basic laws of physics, are fixed. According to the supervenience thesis, therefore, physical laws and causal relations are fundamental; they, and they alone, are ultimately responsible for the causal/nomic structure of the world. But this conclusion does not comport comfortably with the claim that the special sciences are autonomous vis-à-vis basic physics. For if the laws and causal relations obtaining at the basic physical level determine all higher-level causal relations and laws, it should be possible in principle, or at least so it seems, to formulate explanations of higher-level laws and phenomena within the physical domain. If the world works the way it does because the physical world works the way it does, why is it not possible to explain everything in terms of how the physical world works?

Some will challenge this reasoning. They will argue that for X to determineY is one thing, but that for X to explain or make intelligible why Y occurs is quite another. Pain emerges whenever C-fibers are firing, and this may well be a lawlike correlation. But the correlation is "brute": it is not possible to explain why pain, rather than tickle or itch, emerges when C-fibers fire, or why pain emerges from C-fiber excitation but not from other kinds of neural activity. Nor do we seem able to explain why any conscious states should emerge from neural processes. For the emergentists, then, although all higher-level facts are determined by lower-level physical facts, the latter are powerless to explain the former. The world may be a fundamentally physical world, but it may well include physically inexplicable facts.

Whether and how the functionalist can resist the reductionist pressure is less clear. Suppose, as functionalism has it, that being in mental state M is to be in some physical state or other meeting a certain causal specification D. It would seem then that we could easily explain why something is in M by pointing out that it is in P and that P meets causal specification D -- namely that P is a realizer of M. And given the functional characterization of M, it seems to follow that the causal powers of a given instance of M are just the causal powers of its realizer P on this occasion. Thus, if it is a special-science law that M-events cause M*-events, that must be so because each of M's physical realizers causes a physical realizer of M*. In this way, special-science laws would seem reductively explainable in terms of laws governing the realizers of the special-science properties involved.

"Materialism" is often used interchangeably with "physicalism." However, there are some subtle differences between these terms, the most salient of which is that physicalism indicates acknowledgment that something like current physics is the ultimate explanatory theory of all the facts, whereas materialism is not necessarily allied with the success of physics as a basic explanatory theory of the world.

See also

Additional links

-- Jaegwon Kim


Armstrong, D. M. (1968). A Materialist Theory of Mind. New York: Humanities Press.

Block, N., and J. A. Fodor. (1972). What psychological states are not. Philosophical Review 81:159-181.

Churchland, P. M. (1981). Eliminative materialism and the propositional attitudes. Journal of Philosophy 78:68-90.

Davidson, D. (1970). Mental events. In L. Foster and J. W. Swanson, Eds., Experience and Theory. Amherst, MA: University of Massachusetts Press, pp. 79-101.

Feigl, H. (1958). The "mental" and the "physical." Minnesota Studies in the Philosophy of Science 2:370-497.

Fodor, J. A. (1974). Special sciences, or the disunity of science as a working hypothesis. Synthese 28:97-115.

Hellman, G., and F. Thompson. (1975). Physicalism: Ontology, determination, and reduction. Journal of Philosophy 72:551-564.

Horgan, T. (1982). Supervenience and microphysics. Pacific Philosophical Quarterly 63:29-43.

Kim, J. (1984). Concepts of supervenience. Philosophy and Phenomenological Research 45:153-176.

Kim, J. (1992). Multiple realization and the metaphysics of reduction. Philosophy and Phenomenological Research 52:1-26.

Lewis, D. (1972). Psychophysical and theoretical identifications. Australasian Journal of Philosophy 50:249-258.

McLaughlin, B. (1992). The rise and fall of British emergentism. In A. Beckermann, H. Flohr, and J. Kim, Eds., Emergence or Reduction. Berlin: De Gruyter, pp. 49-93.

Morgan, C. L. (1923). Emergent Evolution. London: William and Norgate.

Putnam, H. (1967). Psychological predicates. In W. H. Capitan and D. D. Merrill, Eds., Art, Mind, and Religion. Pittsburgh, PA: University of Pittsburgh Press, pp. 37-48.

Smart, J. J. C. (1959). Sensations and brain processes. Philosophical Review 68:141-156.

Sperry, R. W. (1969). A modified concept of consciousness. Psychological Review 76:532-536.

Further Readings

Broad, C. D. (1925). The Mind and Its Place in Nature. London: Routledge and Kegan Paul.

Kim, J. (1989). The myth of nonreductive materialism. Proceedings and Addresses of the American Philosophical Association 63:31-47.

Levine, J. (1983). Materialism and qualia: The explanatory gap. Pacific Philosophical Quarterly 64:354-361.

McLaughlin, B. (1989). Type epiphenomenalism, type dualism, and the causal priority of the physical. Philosophical Perspectives 3:109-136.

Moser, P. K., and J. D. Trout, Eds. (1995). Contemporary Materialism. London: Routledge.

Poland, J. (1994). Physicalism. Oxford: Clarendon Press.

Rosenthal, D. M., Ed. (1991). The Nature of Mind. New York: Oxford University Press.