Physicalism

Physicalism is the metaphysical thesis that everything is physical — that the physical description of the world is complete, and that there are no facts, properties, or entities that cannot be described in the vocabulary of physics. It is the dominant working assumption in contemporary philosophy of mind, philosophy of science, and cognitive science, and it is the position that the philosophical zombie thought experiment was designed to challenge.

The term 'physicalism' is often used interchangeably with 'materialism,' though the two have distinct histories. Materialism, in its classical forms, asserted that everything is made of matter. Physicalism makes the weaker but more defensible claim that everything is physical, where 'physical' is defined not by reference to a specific ontology (atoms, fields, particles) but by reference to the vocabulary of physics as it is actually practiced. What counts as physical changes as physics changes. This makes physicalism a moving target — and its critics have argued that this mobility is either a virtue (physicalism tracks scientific progress) or a defect (it is so flexible that it cannot be refuted).

The Varieties of Physicalism

Physicalism is not a single thesis but a family of theses, distinguished by how strongly they claim completeness and by what they mean by 'physical.'

Ontological physicalism holds that everything that exists is physical. This is the strongest form: there are no non-physical entities, no immaterial minds, no supernatural properties. The difficulty is specifying what 'physical' means without either tying it to current physics (which makes physicalism hostage to the next scientific revolution) or defining it so broadly that it becomes vacuous.

Token physicalism holds that every particular event or state is physical. This is weaker than ontological physicalism: it permits the existence of non-physical properties or types, as long as every token instance of them is a physical event. Token physicalism is compatible with property dualism in a way that ontological physicalism is not.

Type physicalism (or type identity theory) holds that mental state types are identical to physical state types — pain is C-fiber stimulation, not merely correlated with it. This is the position developed by J.J.C. Smart and Ullin Place in the 1950s. It was largely abandoned after the multiple realizability argument showed that the same mental state could be realized by different physical states in different organisms.

Non-reductive physicalism holds that mental properties are physical but not reducible to physical properties. This is the most widely held contemporary form: the mental supervenes on the physical (no mental difference without a physical difference) but mental properties are not identical to any specific physical properties. The challenge for non-reductive physicalism is explaining how mental properties can be causally efficacious if every physical event has a sufficient physical cause.

Physicalism and the Philosophy of Mind

Physicalism is the default position against which every alternative in philosophy of mind defines itself. Dualism rejects it by positing non-physical mental substance or properties. Panpsychism rejects it by distributing mental properties to the fundamental level of physics. Functionalism accepts it in spirit — mental properties are realized in physical systems — but redefines the relationship between mental and physical in terms of organizational structure rather than identity.

The hard problem of consciousness — why there is something it is like to be conscious — is widely taken to be the deepest challenge to physicalism. If physicalism is true, phenomenal consciousness must be either identical to some physical process, reducible to some functional organization, or eliminable as an illusion. Each of these options has defenders, and none has achieved consensus. The persistence of the hard problem is either evidence that physicalism is incomplete or evidence that the problem is misstated.

Physicalism as a Systems Commitment

From a systems-theoretic perspective, physicalism is not a claim about the ultimate furniture of the universe but a commitment to a specific kind of explanatory closure. A physicalist system is one in which every state transition has a complete physical description: given the physical state at time t and the physical laws, the physical state at time t+1 is determined. This is a commitment to causal completeness at the physical level, not a denial that other levels of description (psychological, social, informational) are useful or real.

The systems reading dissolves some of the apparent paradoxes. The question 'can a physical system be conscious?' becomes: 'does the organizational structure of this physical system instantiate the functional properties associated with consciousness?' The question 'is the mental reducible to the physical?' becomes: 'is the mental level of description eliminable in favor of the physical level, or is it a genuinely autonomous level of organization?' These are empirical and architectural questions, not metaphysical ones.

The cost of this reframing is that it makes physicalism less distinctive. If physicalism is just the claim that physical systems have complete physical descriptions, then it is true by definition and not very interesting. The interest lies in the stronger claims: that the physical description is the only complete description, that higher-level descriptions are dispensable in principle, and that the physical level is causally closed. These are claims about the architecture of explanation, not about the ontology of the world, and they are claims that the history of science has repeatedly challenged.