Materialism and physicalism

What it is

Materialism is the claim that everything which exists is material — that there is no soul, no spirit, no immaterial mind, no realm above or behind the physical one.

Most philosophers now prefer the word physicalism, and the reason matters. "Matter" suggests little bits of stuff, and physics stopped describing the world that way a century ago. Fields, energy, spacetime curvature and quantum states are not matter in any everyday sense, but a physicalist counts them all as physical. The claim is not "everything is made of tiny billiard balls". It is that everything there is falls within the subject matter of a completed physics, with nothing left over.

The position comes in strengths:

• Reductive physicalism — mental states just are brain states, in the way that lightning just is an electrical discharge
• Non-reductive physicalism — the mental depends entirely on the physical without being reducible to it, the way a piece of software depends on hardware
• Eliminative materialism — the strongest version, holding that our everyday mental vocabulary (beliefs, desires) is simply a bad theory that a mature neuroscience will discard, the way chemistry discarded phlogiston

Where it came from

It is one of the oldest positions in philosophy and it did not begin in a laboratory.

The Greek atomists — Leucippus and Democritus in the fifth century BC — held that reality consists of indivisible particles moving in void, and nothing else. Epicurus built an ethics on it, and the Roman poet Lucretius set the whole system out in verse in De Rerum Natura, explicitly to free readers from the fear of gods and of death. That is worth registering: the ancestor of modern materialism was written as a consolation, not as a debunking.

The view went quiet through the Christian centuries and returned with Hobbes in the seventeenth, then gathered force through the nineteenth as the sciences began explaining one previously mysterious domain after another.

Its modern academic form was largely built in the 1950s, and — a detail worth an Australian noting — substantially in Australia. U.T. Place, J.J.C. Smart and D.M. Armstrong developed the identity theory of mind, the claim that sensations are literally identical to brain processes, so thoroughly at Adelaide and Sydney that the position is still sometimes called Australian materialism.

How it is used

In practice, physicalism does two quite different jobs, and conflating them causes a great deal of pointless argument.

As a working method, it is what every laboratory runs on: look for physical causes, because that is what the tools can find and because it has worked spectacularly well. This is methodological naturalism, and it is a research strategy rather than a belief about reality. A scientist can use it all day and go to church on Sunday without inconsistency.

As a claim about what exists, it is a metaphysical position — metaphysical naturalism — which is not established by science but assumed or argued for philosophically.

Almost every dispute on this site between a spiritual claim and "what science says" turns out, on inspection, to be a dispute about whether the success of the first licenses the second. It usually is not made explicit, by either side.

What we can and cannot say

We can say the track record is extraordinary. Domain after domain that once seemed to require something extra — life, heat, disease, inheritance, lightning — turned out to be physical all the way down. Vitalism, the belief that living things contain a non-physical life force, was a serious scientific position within living memory and is now gone. Anyone betting against physicalism is betting against a long unbroken run.

We can also say it is not a proven fact, and that its own defenders know this. Two problems stand out.

The first is definitional, and it is sharp. Hempel's dilemma asks what "physical" means. If it means what current physics says, then physicalism is almost certainly false, since current physics is incomplete. If it means what a completed future physics will say, then the claim has no content yet, because nobody knows what that will include. Physicalists have answers, but none is universally accepted.

The second is consciousness. Every other reduction explained a function — what something does. Experience does not obviously look like a function, and the gap between describing what a brain does and explaining why there is something it is like to be one has not closed. That is treated separately in the hard problem of consciousness.

What we cannot say is that physicalism is obviously true or obviously false. It is the most successful research programme in intellectual history with one large unresolved problem at its centre, which is a description that will not satisfy anyone hoping this site will pick a side.

Further reading

• Lucretius, De Rerum Natura — the ancient statement, and still readable.
• J.J.C. Smart, "Sensations and Brain Processes" (1959) — the founding paper of the modern identity theory.
• Daniel Stoljar, "Physicalism", Stanford Encyclopedia of Philosophy — free, thorough, and unusually clear on the definitional problem.