Author: Bruce Long

Discipline: Philosophy

Subdiscipline: Metaphysics (specifically ontology)

Categories: Ontology, Metaphysics, Metametaphysics, Philosophy of Science

Philosopher(s): Frank Jackson, Willard Van Ormand Quine, David Papineau, Michael Devitt, Jonathan Schaffer .

Relevant Work: (Refer to Amazon recommended reading at bottom of article.)

Physicalism is a commitment to the idea that all that exists in the world as real – all of its existents – are physical or reduce to physical things or structure(s). Determining what is real and how it exists is part of the study of science and of the philosophical discipline of ontology, which latter is a large part of the discipline of science. When discussing ontology, philosophers often speak of the furniture of the universe, or sometimes use phrases and terms like the “roster of existents” (Schaffer, J., 2009. Existents are just things that exist.)

In fact, this ontological position or ontology is very old. Ancient Greek philosophers like Thales, Anaxagoras (ca. 500 BC – 428 BC), Epicurus and Democritus prefigure later materialists. The Latin poem De Rerum Natura by Lucretius (ca. 99 BC – ca. 55 BC) reflects the mechanistic philosophy of Democritus and Epicurus (Source: Wiki).

Anaxagoras was a Greek Materialist. He is the earliest known recorded scholar to espouse a Heliocentric model of the solar system.

Thucydides: Early Greek Materialist Political Philosopher

Physicalism is generally understood by philosophers to be synonymous with metaphysical materialism. It is surprisingly difficult to produce a non-circular and non-question-begging* conception of physicalism. This is partly because if physicalism is true and everything in the universe is in fact physical, then it is not clear what this even means since one has to first state explicitly what the property of being physical is or what means to say something is physical.

It is arguably not clear, despite commonsense assertions and intuition, what this does mean.

Another challenge for philosophers (and of course scientists) is to determine what is real and what is not real. Science, and especially physics, has long since taken over the role of identifying and classifying the physical structures in the universe. However, recent cosmology and quantum physics has seen a revival in philosophical ontological questions in relation to the fundamental nature of physical reality. These questions have to do with how things exist, what their scope is, whether they can exist in other dimensions, and in what ways they can be regarded as physical and how.

A commonly known way to classify something as physical that arose from the practice of the physical sciences states roughly that if it cannot be ‘sprayed’ then it is not physical. Physicists generally have a ready guide to the physical available in experimentation. Since all of the apparatus and equipment used for experimentation is conventionally and non-controversially physical, and since the phenomena used for measurement of and interaction with the objects of experimentation (radiation, particle emissions, various natural kinds of energy) So returning to the question of what is real, the physicist can answer that something is physically real if they can experiment upon it with physical apparatus that manipulates and operates with known physical phenomena.

Reductive and Non-Redcutive Physicalism, and Naturalism

However, answering questions about what is and is not physical and physically real on the above stated basis does not constitute physicalism, but is only part of the project of physicalist science and philosophy. Physicalism is an assertion about what does and does not exist – about what is an is not real – in the world. The strongest kind of Physicalism is a commitment to the principle that everything that can be classified as real either is physical or reduces to or is made up of physical things or structures. Philosophers sometimes call this reductive physicalism. There is a less stringent conception of physicalism that is called non-reductive physicalism. This kind of physicalism does not require everything that exists – everything in the ontology (the roster of existents or the furniture of the universe) – to be or to reduce to physical structure(s) and/or entities.

Brutalist Architecture. The Aesthetic of physicalism in Concrete. Brutalism has an aesthetic heritage in Soviet Marxist materialism also.

Non reductive physicalists are metaphysical materialists and generally also naturalist or naturalistic about ontology. However, they allow for the reality of things that are based upon or grounded in physical things and structures in special ways, but that cannot themselves be easily thought of as physical. These ways of being real and based upon physical structure and things without being physical include emergence, supervenience, and relations. Supervenience is the principle that if some thing B existentially depends upon some thing A (depends for its existenc upon A or would not exist without A), then some detectable change in B necessarily indicates some change in A. The trick is that while and the thing A is non-controverisially physical, B is not necessarily physical. This is supervenince. Emergence is sometimes described as a consequence of supervenience – but not always. Emergence is the idea that some phenomena in nature cannot be reduced to physical elements and structures – dynamical or otherwise (See Mitchell, S. 2010 for a good discussion and argument in favour of emergence and non-reductive physicalism.)

Naturalism is the idea that there exists only natural things, and that there is nothing that exists outside of nature. Nature is taken to be bounded in various important ways. One principle that is applied is that of physical causal closure. Roughly put, it is the idea that everything in nature is subject to physical causal processes and there is nothing non-physical that can cause anything to happen in nature. A negative statement of naturalism – one that defines it according to what it denies or does not allow – is that it is the rejection of the reality of any putative supernatural existents.

Physicalist philosophers are either reductive or non-reductive in their ontological commitments, and are always naturalistic (provided the conception of naturalism is not too liberal – see the text by DeCaro for a set of papers that question the definition of naturalism.) Emergence may or may not be something that either reductive or non-reductive physicalists accept as real, since emergence arguably be defined in physicalist informationist terms (beyond the scope of this primer.)

Formal Statements of Physicalism

Frank Jackson has offered a statement of physicalism as follows:

Any world which is a minimal physical duplicate of our world is a duplicate simpliciter of our world

In simple terms – if you copied all of the physical stuff in our world, you would not have left any real thing out. You would reproduce everything in the ontology – all of the existents in the roster of existents.

Some Consequences of Physicalism

The Philosophy of Mind and The Explanatory Gap

If physicalism is true, and especially reductive physicalism, then questions about the nature of the mind – how the mind exists and what it actually is – must be answered on physicalist terms. The view that minds – including the thoughts, concepts, memories, and imaginings in them all reduce to physical structures (associated with energy, matter, and perhaps other physical substances that are less well understood by or as yet undiscovered or unclassified by science) and interactions between physical substances – usually resulting in some kind of natural information processing which itself reduces to physical dynamics of some kind.

There are numerous physicalist conceptions of mind, and several non-phsyicalist and non-reductionist theories of mind (of how the mind exists and what it is.) The idea of the explanatory gap is associated with the latter. It is the principle that, should an entire physical theory and description of the universe be produced (by some very successful scientists) then that theory will still not explain how the mind works and what it is. The explanatory gap asserts that there will always be an in principle failure to explain the mind on a physicalist basis.

(See Papineau, D, 2004 for a defence of physicalist theories of mind in the face of claims about the truth of the explanatory gap.)

Physical Determinism

Determinism is an enormous topic in philosophy, and there is not room for a full discussion here. Put simply, determinism is the idea that everything that exists and every state of affairs has been fully determined by the state of the physical universe at an earlier point in time. An arguably unpleasant upshot of this is that human actions and behaviour have all been determined by causal forces and interactions that are not within the control of any human agency. In other words, free will is an illusion. Put otherwise – when you make a decision, you only think you have made it based upon your agency and free will – but in fact you were always going to make that decision as determined fully by the prior state of the physical universe. This is not the same this as fatalism, with which it is often confused. Fatalism ignores the state of the universe and sets the outcomes in the future independently of it and everything else. Determinised is instead about the state of the physical universe as the only real determiner of all future states of the universe – including the states of all subsuystems and dynamical systems in it (such as human brains.)

If physicalism is true, then any escape from determinism will need to come from the physical universe itself. Some possibilities for this include quantum indeterminacy and chaos theory.


1. Schaffer, J., (2009) On What Grounds What in Metametaphysics, eds. Chalmers, Manley, and Wasserman Oxford University Press: Oxford, 347-383. 2. Jackson, F., (1993) Armchair Metaphysics in Philosophy in Mind J. Hawthorne and M. Michael (eds.) Amsterdam: Kluwer. (Jackson’s From Metaphysics to Ethics is a more recent refernce – see below.)

2. Connor, W. R., & Societies, A. C. of L. (1987). Thucydides (Limit). Ewing;Princeton; Princeton University Press.

*In philosophy, question begging is not begging for a question to be asked. It is the use of a conclusion that one wants to prove as one of the premises or reasons given in the proof. The Latin term is petitio principii and it is classified as a fallacy of vacuity.

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