physicalism in the widest sense of the term, materialism applied to the question of the nature of mind. So construed, physicalism is the thesis – call it ontological physicalism – that whatever exists or occurs is ultimately constituted out of physical entities. But sometimes ‘physicalism’ is used to refer to the thesis that whatever exists or occurs can be completely described in the vocabulary of physics. Such a view goes with either reductionism or eliminativism about the mental. Here reductionism is the view that psychological explanations, including explanations in terms of ‘folk-psychological’ concepts such as those of belief and desire, are reducible to explanations formulable in a physical vocabulary, which in turn would imply that entities referred to in psychological explanations can be fully described in physical terms; and elminativism is the view that nothing corresponds to the terms in psychological explanations, and that the only correct explanations are in physical terms. The term ‘physicalism’ appears to have originated in the Vienna Circle, and the reductionist version initially favored there was a version of behaviorism: psychological statements were held to be translatable into behavioral statements, mainly hypothetical conditionals, expressible in a physical vocabulary. The psychophysical identity theory held by Herbert Feigl, Smart, and others, sometimes called type physicalism, is reductionist in a somewhat different sense. This holds that mental states and events are identical with neurophysiological states and events. While it denies that there can be analytic, meaning-preserving translations of mental statements into physicalistic ones, it holds that by means of synthetic ‘bridge laws,’ identifying mental types with physical ones, mental statements can in principle be translated into physicalistic ones with which they are at least nomologically equivalent (if the terms in the bridge laws are rigid designators, the equivalence will be necessary). The possibility of such a translation is typically denied by functionalist accounts of mind, on the grounds that the same mental state may have indefinitely many different physical realizations, and sometimes on the grounds that it is logically possible, even if it never happens, that mental states should be realized non-physically.
In his classic paper ‘The ‘mental’ and the ‘physical’ ‘ (1958), Feigl distinguishes two senses of ‘physical’: ‘physical1’ and ‘physical2’. ‘Physical1’ is practically synonymous with ‘scientific’, applying to whatever is ‘an essential part of the coherent and adequate descriptive and explanatory account of the spatiotemporal world.’ ‘Physical2’ refers to ‘the type of concepts and laws which suffice in principle for the explanation and prediction of inorganic processes.’ (It would seem that if Cartesian dualism were true, supposing that possible, then once an integrated science of the interaction of immaterial souls and material bodies had been developed, concepts for describing the former would count as physical1.) Construed as an ontological doctrine, physicalism says that whatever exists or occurs is entirely constituted out of those entities that constitute inorganic things and processes. Construed as a reductionist or elminativist thesis about description and explanation, it is the claim that a vocabulary adequate for describing and explaining inorganic things and processes is adequate for describing and explaining whatever exists.
While the second of these theses seems to imply the first, the first does not imply the second. It can be questioned whether the notion of a ‘full’ description of what exists makes sense. And many ontological physicalists (materialists) hold that a reduction to explanations couched in the terminology of physics is impossible, not only in the case of psychological explanations but also in the case of explanations couched in the terminology of such special sciences as biology. Their objection to such reduction is not merely that a purely physical description of (e.g.) biological or psychological phenomena would be unwieldy; it is that such descriptions necessarily miss important laws and generalizations, ones that can only be formulated in terms of biological, psychological, etc., concepts. If ontological physicalists (materialists) are not committed to the reducibility of psychology to physics, neither are they committed to any sort of identity theory claiming that entities picked out by mental or psychological descriptions are identical to entities fully characterizable by physical descriptions. As already noted, materialists who are functionalists deny that there are typetype identities between mental entities and physical ones. And some deny that materialists are even committed to token-token identities, claiming that any psychological event could have had a different physical composition and so is not identical to any event individuated in terms of a purely physical taxonomy. See also NATURALISM , PHILOSOPHY OF MIND , REDUCTION , UNITY OF SCIENC. S.Sho.