Physicalism, a term coined by Rudolf Carnap, is the thesis, or doctrine, that all knowledge is reducible, at least theoretically, to statements of physics. The human and social sciences, just like the natural sciences, which each have their specific vocabulary and concepts, could be transcribed into the language of physics. In the first definition of physicalism, which is that of the Vienna Circle, such a language consists of a set of statements relating to physical objects, their properties as well as their spatio-temporal characteristics. This language is reduced to protocols or reports of experience and to logical statements which only have meaning in relation to possible objects.
In addition, physicalism supports the thesis that there is no philosophical knowledge made up of theses of its own, which are distinct and independent from scientific theses, and it conceives of philosophical activity as an extension of scientific activity, first as a research on the structures of knowledge, then as an exercise in clarifying and interpreting scientific knowledge.
Physicalism has also been called “theory of the unity of science” or “theory of unitary science”.
The physicalism of the Vienna Circle
The physicalism of the Vienna Circle seems to have found its first formulation with the sociologist and economist Otto Neurath, who developed at the turn of the 1930s a true physicalist conception of the language of science. For him it is appropriate to distinguish at least two meanings of physicalism: a methodological meaning and an ontological meaning. Neurath clearly leans for the former.
For him, physicalism is not a doctrine which explains the ultimate nature of objects, because that would be to do metaphysics, but a methodological principle of description of natural objects and processes, including human societies and psychic processes, in spatio-temporal terms. Thanks to the unity of terminology and concepts, the physicalist language makes it possible to connect all the statements and to use statements from several disciplines, even apparently distant from each other, to predict complex phenomena. Neurath mentions as an example a forest fire. In the forecast of its evolution enter statements of geography as well as meteorology and botany; moreover, if one wants to predict the behavior of an exotic tribe in the face of fire, it is necessary to have recourse to statements from sociology, ethnology and psychology. However, in the forecasts, all these statements must be able to be combined, all must therefore use the same language and a unitary terminology. The ultimate aim of physicalism is the construction of a unitary science on the basis of this language.
At the heart of physicalism is the problem of describing psychic events in physicalist terms (see the body-mind problem). In accordance with the physicalist thesis, terminology that speaks of consciousness must be replaced by the description of neuro-physiological processes or observable bodily events like gestures and words. Along with the soul and other props of old metaphysics, Neurath also dismisses consciousness. Properly psychological concepts must be replaced or eliminated from the explanation of the behavior of human beings and other living organisms (cf. eliminativism):
“Not only is the mind no longer a product of matter, but one can no longer even formulate in a meaningful way the expressions “spirit” or “spiritual process” nor speaking of spirit; in their place enter, at a fundamental level, formulations in which enter only spatio-temporal relations […]. The question of “spirit” or “matter” is resolved by the disappearance of the doctrine of the spirit; only the doctrine of “matter” remains, namely physics. What is given as science of reality cannot be anything other than physics.”
Physics, in the broad sense, becomes an encompassing discipline, a transdiscipline whose terminology claims universality. We will then speak of homogeneous reductions of all fields of knowledge to that of physics.
Ontological or metaphysical physicalism refers to the set of ontological doctrines which hold that all entities that exist in the world are ultimately physical entities which can or could, in principle, be described by the physical sciences, and whose causal interactions are completely governed by physical laws.
This form of physicalism corresponds to the contemporary form of materialism and was first developed as a philosophical system by W. V. O. Quine from the 1950s. It explicitly opposes Cartesian-type dualism and attempts to reconcile materialism with mental concepts falling within our common conception of the mind. The metaphysical thesis of physicalism that there are only physical entities or properties implies that mental entities, if they exist, do not have a particular ontological status. This thesis is today the subject of a fairly large consensus within analytical metaphysics and the philosophy of mind, but it also has its opponents among contemporary philosophers of the mind who are authoritative like Thomas Nagel. or David Chalmers.
A particularly strong version of physicalism has been proposed by U. T. Place and J. J. C. Smart to resolve the question of the nature of mind within a materialistic framework. This is the so-called “type identity” theory or psychophysical identity theory (also called simply “materialism” in the restrictive sense given to it in philosophy of mind). It is largely inspired by the model of intertheoretical reduction in science. Psychology is conceived as a high level theory in principle reducible to a physico-chemical theory of brain states and processes. The types of entities and properties that psychology postulates are identified with types of brain entities or processes. This identification consists in systematically putting in correspondence the psychological concepts with those of neurobiology, and, by this means, to relate them to the even more fundamental concepts of physics-chemistry. The nature of mental entities and their causal powers cannot be explained without this at least theoretical reduction and identification of psychology with a discourse referring to the structure and activity of the brain.
Eliminativist materialism is an even more radical version of physicalism which questions the possibility of such a reduction and therefore proposes that mental concepts be eliminated from the vocabulary of the physicalist language. According to Paul and Patricia Churchland, our mental concepts are derived from our everyday naive psychology, conceived as a proto-scientific, obsolete and largely erroneous empirical theory. The concepts and statements of this psychology, because they differ radically from those of physics, chemistry and biology (especially neurosciences), cannot be translated term by term into a more fundamental scientific language, such as are for example the concepts defined by neurobiology. Therefore, they must be eliminated and replaced by the scientifically valid and reducible categories of neuroscience.
According to Stephen Stich, the categories of naive psychology must be redefined in such a way as to be reducible to neurobiological properties or entities, which amounts to adopting an approach of “revision” rather than elimination with regard to the mentalist vocabulary.
Since the 1970s, many philosophers of the mind have argued that the compatibility of psychological discourse with physicalism does not require a reduction of psychology to the physical sciences.
(For functionalism, the human brain and the mind, are in the same relationship as the computer and the computer program it executes.)
The anomalous monism proposed by Donald Davidson admits that every particular mental process (“event”) is identical to a particular physical process, in other words, it admits that mental concepts refer to the same events as physical concepts. He thus adopts the theory known as the “identity of occurrences” (token identity). Davidson considers, however, that mental concepts are irreducible to physical concepts. Indeed, there are strict causal laws only at the physical level. However, psychological descriptions and explanations of behavior operate within an essentially normative and holistic framework that appeals to “reasons” or “motives” for action rather than to causes of behavior. If, for Davidson, mental processes are indeed the causes of physical processes, it is only insofar as they are physical events and not by virtue of the properties that mental concepts describe. This amounts to excluding the mental properties of physicalist ontology.
Functionalism, in particular in the version given by Jerry Fodor and Hilary Putnam, offers a form of non-reductionist physicalism, also based on the token identity. He considers that what defines mental properties and types of mental states is not their physical constitution but their function or “causal role” within a physical system interacting with its environment. The internal states of a complex artificial machine, in particular a computer, can thus theoretically play the same role as those of the human brain.
The notion of supervenience, developed in particular by Jaegwon Kim, following Davidson, has often been used to specify the nature of the dependency links between mental and physical properties, in a non-reductionist physicalist framework.