Physicalism

Physics as the Definition of the Universe§

Physicalism is the ontological thesis that everything that exists is ultimately dependent upon, and can be accounted for by, the fundamental laws and entities of physics. So on this view a thought is, in the end, nothing but physics — no less than a falling rock is.

Dynamical part§

Physicalism ties what exists to whatever physics says exists. On one view, that makes it a moving target: whenever a frontier theory — the current best, most cutting-edge account — breaks down under new observations, the very view of "what exists" has to shift with it. Not everyone accepts this — the camps below split on precisely this point, and one of them holds that the old picture survives as an approximation rather than being replaced.

But follow the thought and a deeper problem appears. For physicalism to be settled as absolute truth, we would have to know that physics is finished — the inventory complete, with nothing further to add. History gives no encouragement here. Caloric — the once-standard theory that heat was a fluid — was simply refuted. Newton's absolute space went more quietly: it was never refuted at all, but abandoned by choice, on grounds of simplicity rather than evidence, exactly as the Ontology section's underdetermination case describes. Either way the inventory did not hold still.

The track record, though, is not the real obstacle. By the standard the Epistemology section already set, "everything is physical" is a universal claim, and universal claims are never verified — only ever left not-yet-falsified. Just as no number of white swans could prove "all swans are white," no accumulation of confirmed physics could prove "everything is physical": there is always one more case left unchecked. Ordinary physical laws carry the same limitation, of course. The difference is that physics holds them tentatively, as working best guesses, while physicalism needs its universal to be settled.

So even a physics that happened to be complete could never be certified as complete. Unlike the purely interpretive questions of the Ontology section, which no observation could touch at all, this one at least looks checkable — though one of the two camps below will turn out to land back in that untouchable category anyway. It is only when you ask what would count as checking it that the floor gives way. The completion physicalism would need is therefore not merely far off. Certified completion is unreachable in principle — not because physics cannot finish, but because we could never be entitled to say that it had.

Camps§

This has produced an ongoing, internal schism within the philosophy of science, between the two camps below.

Camp 1: The "Future Physics" Camp (The Broad View)§

They accept that physics will change, and refuse to bet the thesis on any particular stage of it. They argue that "physical" means whatever entities a completed, ideal physics would require in order to explain the world, whatever those turn out to be.

Camp 2: The "Current Physics" Camp (The Strict View)§

They tie physicalism to our current, best-tested theories — the Standard Model of particle physics (our account of the subatomic particles and three of the four fundamental forces between them, gravity being the exception) and General Relativity (Einstein's account of gravity as the shape of spacetime). Any future physics, they argue, will still retain these core frameworks as approximations.

Hempel’s Dilemma§

Physics-the-method never hands you a finished inventory — which is exactly why "which inventory?" becomes a live question at all. Philosopher Carl Hempel argued that physicalists cannot clearly define what "physics" actually means when they say "everything is physical" — because either camp of physicalism has crucial flaws.

Current Physics§

If physicalism means "everything is explained by today's physics" (Camp 2 above), it is certainly false. We already know our current theories are incomplete — for example, quantum mechanics and general relativity each work superbly in their own domain, yet they cannot be combined into one consistent account: pushed together where gravity is strong and distances are tiny — inside a black hole, or at the first instant of the universe — the mathematics breaks down into meaningless infinities rather than yielding any prediction at all. Camp 2's reply — that future physics will keep these frameworks as approximations — does not rescue the claim: to survive as an approximation is precisely to concede that today's physics is not the finished account, which is what the strict reading needs it to be.

Ideal Future Physics§

If it means "everything explained by some ideal, future physics" (Camp 1 above), then the statement becomes empty. If future physics discovers God, ghosts, or a non-physical mind, they simply get labeled as "physical." The definition loses its meaning. This is the same rigged-unfalsifiability move the Epistemology section warned about: a claim elastic enough to absorb any possible counter-evidence has stopped telling you anything about the world.

Examples of Physicalism§

Hempel's Dilemma is about what the word "physical" can be made to mean. Set that aside now and ask what physicalists actually claim in practice. The test case is always the mind: it is the hardest thing in the world to account for as "nothing but physics," so it is where the thesis either holds or breaks.

Type Identity Theory§

J.J.C. Smart and others hold that specific mental states literally are specific brain states — not caused by them, not merely accompanied by them, but identical to them. The stock example is that pain just is the firing of C-fibres, the nerve fibres that carry slow, burning pain.

Take that example as a placeholder rather than a result. C-fibres are real. But they are peripheral nerves, not brain structures, so this particular pairing cannot be the literal identity the theory is after. The thesis is still meant literally; it is the stock illustration that is loose. Read "C-fibres" as standing in for "whatever brain state turns out to realise pain, whichever one it proves to be." (Whether philosophers always used it that knowingly, or were simply repeating a piece of sloppy folk neuroscience, is itself disputed.) What the example preserves is the shape of the claim: whatever pain turns out to be, it will be some physical state and nothing further. Smart's own analogy was a discovery of exactly that shape — lightning turning out to be electrical discharge, rather than merely to arrive alongside it.

Eliminative Materialism§

Paul and Patricia Churchland argue that our common-sense "folk psychology" — beliefs, desires, intentions — is not a rough description of something real, but an inaccurate theory outright: one that a mature neuroscience will eventually replace rather than refine.

Their model for this is caloric, met above: the idea that heat was an invisible, weightless fluid flowing from hot bodies into cold ones, abandoned once heat turned out to be nothing but the motion of molecules. Nobody kept caloric on afterwards as a useful approximation — there was no fluid, and the word simply stopped referring to anything. On this view "belief" and "desire" are headed the same way.

Anti-Physicalism§

Both positions above say the mind is nothing but the physical, described either correctly or badly. The three arguments below press a single objection against them: something about conscious experience seems to slip through any purely physical description, however complete.

Mary’s Room§

Proposed by Frank Jackson — it is also called the knowledge argument — this experiment involves Mary, a brilliant scientist who investigates the world from a black-and-white room. She learns every physical fact about the neurophysiology of vision but has never seen color. When she finally steps outside and sees a red apple, she learns something new: what it is like to experience the color red. This implies physical facts alone cannot fully describe conscious experience.

The Explanatory Gap§

Coined by Joseph Levine, this concept highlights that even if we map every neural pathway involved in an experience, a gap remains in our understanding. Physical sciences excel at explaining functions, structures, and behaviors. However, they fail to explain why or how those physical processes give rise to subjective, felt experiences.

The Philosophical Zombie (The Conceivability Argument)§

Popularized by David Chalmers, this argument asks you to imagine a creature that is physically identical to a human being down to the atom. This "zombie" behaves normally, talks, and reacts to stimuli, but lacks any inner conscious experience or subjective feelings. Because such a being is logically conceivable, the argument runs, conscious experience must be something over and above purely physical facts — though the step from "conceivable" to "genuinely possible" is where most of the fighting happens.

Physicalist takes on Mary's room§

All three replies below agree that Mary learns no new fact — if she did, physicalism would be false. They differ over what she gains instead: a skill she did not have before (Ability), a first-hand encounter with a fact she already knew (Acquaintance), or a new kind of concept for that same old fact (Old Fact, New Concepts).

The Ability Hypothesis§

Mary does not gain new factual knowledge when she sees red; she simply acquires a new practical skill: the ability to recognize, remember, and imagine the color red.

The Acquaintance Hypothesis§

Mary does not learn a new truth about the world; she merely becomes directly acquainted with an old physical property (color processing) via a new, first-person psychological pathway.

Old Fact, New Concepts§

Mary already knew the physical fact objectively, but she now experiences that exact same physical fact using a subjective, phenomenal concept — a way of thinking about something in terms of what it is like from the inside, as opposed to describing it from the outside in the third person. This last pair are close cousins, and where exactly the line between them falls is still argued over: Acquaintance stresses the encounter itself, Old Fact/New Concepts stresses the new mental filing system the encounter hands you.

Property Dualism§

Suppose the physicalist replies above do not quite convince you — but a separate, non-physical soul convinces you even less. There is a middle path.

Two Properties, One Substance§

Property dualism rejects the idea of a separate non-physical soul, agreeing with physicalists that there is only one kind of stuff in the world — physical matter, the brain included. But because it denies that mental properties reduce to physical ones — that they are nothing more than physical properties under another description — philosophers file property dualism as a rival to physicalism rather than a version of it.

Irreducible Features§

It argues that this single physical substance possesses two completely different types of properties: physical properties (like mass and electrical charge) and non-physical mental properties (like consciousness and feelings).

The Emergence View§

When physical matter reaches a high level of biological complexity, like a human brain, subjective mental properties naturally emerge from the system but cannot be reduced down to physics or chemistry. To emerge here means to be a property of a whole system that belongs to none of its parts — wetness is not a property of any single water molecule, only of many together. Consciousness is claimed to be like that, but stronger: not merely hard to derive from the parts, but not derivable from them at all, which is what calling it irreducible above asserts.

Notice what this tour has and has not shown. Type identity and eliminative materialism; the knowledge argument, the explanatory gap, and the zombie beside it; property dualism — every one of them is a dispute about whether physics's inventory can be made to contain us. More neuroscience can settle which brain state is involved; it cannot settle whether that fact is an identity, an elimination, or something further. That is the warning from the top of this section, and the hardest case there is did not overturn it: physics supplies the inventory; what you conclude about what exists is a philosophical claim, and it has to be argued for.