Physicalism
In philosophy, physicalism is the view that "everything is physical", that there is "nothing over and above" the physical,[1] or that everything supervenes on the physical.[2] It is opposed to idealism, according to which the world arises from mind. Physicalism is a form of ontological monism—a "one substance" view of the nature of reality, unlike "two-substance" (mind–body dualism) or "many-substance" (pluralism) views. Both the definition of "physical" and the meaning of physicalism have been debated.
Physicalism is closely related to materialism, and has evolved from materialism with advancements in the physical sciences in explaining observed phenomena. The terms "physicalism" and "materialism" are often used interchangeably, but can be distinguished based on their philosophical implications. Physicalism encompasses matter, but also energy, physical laws, space, time, structure, physical processes, information, state, and forces, among other things, as described by physics and other sciences, as part of the physical in a monistic sense. From a physicalist perspective, even abstract concepts such as mathematics, morality, consciousness, intentionality, and meaning are considered physical entities, although they may consist of a large ontological object and a causally complex structure.[3]
According to a 2009 survey, physicalism is the majority view among philosophers,[4] but there also remains significant opposition to physicalism.
Outside of philosophy, physicalism can also refer to the preference or viewpoint that physics should be considered the best and only way to render truth about the world or reality.[5]
Reductionism and emergentism[edit]
Reductionism[edit]
There are multiple versions of reductionism.[2] In the context of physicalism, the reductions referred to are of a "linguistic" nature, allowing discussions of, say, mental phenomena to be translated into discussions of physics. In one formulation, every concept is analysed in terms of a physical concept. One counterargument to this supposes there may be an additional class of expressions that is non-physical but increases a theory's expressive power.[36] Another version of reductionism is based on the requirement that one theory (mental or physical) be logically derivable from a second.[37]
The combination of reductionism and physicalism is usually called reductive physicalism in the philosophy of mind. The opposite view is non-reductive physicalism. Reductive physicalism is the view that mental states are both nothing over and above physical states and reducible to physical states. One version of reductive physicalism is type physicalism, or mind-body identity theory. Type physicalism asserts that "for every actually instantiated property F, there is some physical property G such that F=G".[2] Unlike token physicalism, type physicalism entails supervenience physicalism.
Another common argument against type physicalism is multiple realizability, the possibility that a psychological process (say) could be instantiated by many different neurological processes (even non-neurological processes, in the case of machine or alien intelligence).[34][38] For in this case, the neurological terms translating a psychological term must be disjunctions over the possible instantiations, and it is argued that no physical law can use these disjunctions as terms.[38] Type physicalism was the original target of the multiple realizability argument, and it is not clear that token physicalism is susceptible to objections from multiple realizability.[39]
A priori versus a posteriori physicalism[edit]
Physicalists hold that physicalism is true. A natural question for physicalists, then, is whether the truth of physicalism is deducible a priori from the nature of the physical world (i.e., the inference is justified independently of experience, even though the nature of the physical world can itself only be determined through experience) or can only be deduced a posteriori (i.e., the justification of the inference itself is dependent upon experience). So-called "a priori physicalists" hold that from knowledge of the conjunction of all physical truths, a totality or that's-all truth (to rule out non-physical epiphenomena, and enforce the closure of the physical world), and some primitive indexical truths such as "I am A" and "now is B", the truth of physicalism is knowable a priori.[41] Let "P" stand for the conjunction of all physical truths and laws, "T" for a that's-all truth, "I" for the indexical "centering" truths, and "N" for any [presumably non-physical] truth at the actual world. We can then, using the material conditional "→", represent a priori physicalism as the thesis that PTI → N is knowable a priori.[41] An important wrinkle here is that the concepts in N must be possessed non-deferentially in order for PTI → N to be knowable a priori. The suggestion, then, is that possession of the concepts in the consequent, plus the empirical information in the antecedent is sufficient for the consequent to be knowable a priori.
An "a posteriori physicalist", on the other hand, will reject the claim that PTI → N is knowable a priori. Rather, they would hold that the inference from PTI to N is justified by metaphysical considerations that in turn can be derived from experience. So the claim then is that "PTI and not N" is metaphysically impossible.
One commonly issued challenge to a priori physicalism and to physicalism in general is the "conceivability argument", or zombie argument.[42] At a rough approximation, the conceivability argument runs as follows:
P1) PTI and not Q (where "Q" stands for the conjunction of all truths about consciousness, or some "generic" truth about someone being "phenomenally" conscious [i.e., there is "something it is like"[43] to be a person x] ) is conceivable (i.e., it is not knowable a priori that PTI and not Q is false).
P2) If PTI and not Q is conceivable, then PTI and not Q is metaphysically possible.
P3) If PTI and not Q is metaphysically possible then physicalism is false.
C) Physicalism is false.[44]
Here proposition P3 is a direct application of the supervenience of consciousness, and hence of any supervenience-based version of physicalism: If PTI and not Q is possible, there is some possible world where it is true. This world differs from [the relevant indexing on] our world, where PTIQ is true. But the other world is a minimal physical duplicate of our world, because PT is true there. So there is a possible world which is a minimal physical duplicate of our world, but not a full duplicate; this contradicts the definition of physicalism that we saw above.
Since a priori physicalists hold that PTI → N is a priori, they are committed to denying P1) of the conceivability argument. The a priori physicalist, then, must argue that PTI and not Q, on ideal rational reflection, is incoherent or contradictory.[45]
A posteriori physicalists, on the other hand, generally accept P1) but deny P2)--the move from "conceivability to metaphysical possibility". Some a posteriori physicalists think that unlike the possession of most, if not all other empirical concepts, the possession of consciousness has the special property that the presence of PTI and the absence of consciousness will be conceivable—even though, according to them, it is knowable a posteriori that PTI and not Q is not metaphysically possible. These a posteriori physicalists endorse some version of what Daniel Stoljar (2005) has called "the phenomenal concept strategy".[46] Roughly speaking, the phenomenal concept strategy is a label for those a posteriori physicalists who attempt to show that it is only the concept of consciousness—not the property—that is in some way "special" or sui generis.[47] Other a posteriori physicalists[48] eschew the phenomenal concept strategy, and argue that even ordinary macroscopic truths such as "water covers 60% of the earth's surface" are not knowable a priori from PTI and a non-deferential grasp of the concepts "water" and "earth" et cetera. If this is correct, then we should (arguably) conclude that conceivability does not entail metaphysical possibility, and P2) of the conceivability argument against physicalism is false.[49]
Other views[edit]
Realistic physicalism[edit]
Galen Strawson's realistic physicalism or realistic monism[50] entails panpsychism – or at least micropsychism.[51][52][53] Strawson argues that "many—perhaps most—of those who call themselves physicalists or materialists [are mistakenly] committed to the thesis that physical stuff is, in itself, in its fundamental nature, something wholly and utterly non-experiential... even when they are prepared to admit with Eddington that physical stuff has, in itself, 'a nature capable of manifesting itself as mental activity', i.e. as experience or consciousness".[51] Because experiential phenomena allegedly cannot be emergent from wholly non-experiential phenomena, philosophers are driven to substance dualism, property dualism, eliminative materialism and "all other crazy attempts at wholesale mental-to-non-mental reduction".[51]