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Physicalism (or materialism) holds that everything that exists, including mental states, is ultimately physical or depends entirely on the physical. Mental states are either identical to, or fully determined by, physical states of the brain. Physicalism comes in various strengths: type identity theory, token physicalism, and supervenience physicalism differ in how tightly they tie mental to physical properties. The main challenge for all versions is explaining consciousness — why should physical processes give rise to subjective experience at all?
Study the spectrum from eliminativist physicalism (there are no mental states as folk psychology describes them) through reductive identity theory to non-reductive supervenience. Contrast each with the specific objections it faces — the multiple realizability objection against type identity, the explanatory gap against all versions.
Physicalism is the view that everything that exists is physical — or, more carefully, that everything that exists depends entirely on the physical. Applied to the mind, this means mental states (beliefs, pains, desires, experiences) are either identical to physical brain states or are fully determined by them. Physicalism is the dominant view in contemporary philosophy of mind, in part because it aligns with the success of neuroscience, but also because the main alternative — dualism — faces serious explanatory problems of its own (famously, how does an immaterial mind interact with a physical body?).
Physicalism comes in importantly different strengths. Type identity theory makes the strongest claim: each mental type (pain, belief, desire) is strictly identical to a physical type (a specific pattern of neural activity). This view has theoretical elegance but faces the multiple realizability objection: pain is realized by very different physical mechanisms in humans, octopuses, and conceivably in silicon systems. If that is right, "pain" cannot be identical to any single physical type. Token physicalism responds by weakening the identity claim — each individual mental event is physical, but mental types need not map onto single physical types. Non-reductive physicalism goes further: mental properties are real and causally efficacious but are not definable in purely physical terms, even if everything mental supervenes on the physical.
Supervenience is the key technical notion here. Mental properties supervene on physical properties when there can be no mental difference without a physical difference. If two individuals are physically identical in every way, they must have identical mental states. This is a relatively weak claim — it says the mental is fully determined by the physical — without claiming that mental concepts can be reduced to or eliminated in favor of physical ones. Non-reductive physicalists accept supervenience while denying that reduction is possible or desirable.
The most serious challenge for every form of physicalism is the hard problem of consciousness, a term coined by David Chalmers. Even granting that all mental states are physical or supervene on the physical, an explanatory gap remains: why should any physical process be accompanied by subjective experience — by there being "something it is like" to be in that state? You can give a complete physical description of the brain process involved in seeing red, but that description seems to leave out the felt redness of the experience. Physicalism does not make this gap disappear; it insists that the gap must have a physical explanation, even if we do not yet have one.
Understanding where you stand on physicalism requires holding this distinction clearly: physicalism is an ontological thesis (about what exists), not an epistemological one (about what we can explain). A committed physicalist can acknowledge deep explanatory puzzles about consciousness while maintaining that no non-physical entities will ultimately be needed to explain them. That commitment is what makes physicalism a substantive philosophical position rather than simply a restatement of scientific practice.
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