Physicalism is a philosophical theory holding that everything which exists is no more extensive than its physical properties; that is, that there are no kinds of things other than physical things. The term was coined by Otto Neurath in a series of early twentieth century essays on the subject, in which he wrote:

:"According to physicalism, the language of physics is the universal language of science and, consequently, any knowledge can be brought back to the statements on the physical objects. "

In contemporary philosophy, physicalism is most frequently associated with the mind-body problem in philosophy of mind, regarding which physicalism holds that all that has been ascribed to "mind" is more correctly ascribed to "brain" or the activity of the brain. Physicalism is also called "materialism", but the term "physicalism" is preferable because it has evolved with the physical sciences to incorporate far more sophisticated notions of physicality than matter, for example wave/particle relationships and non-material forces produced by particles.

The related position of methodological naturalism says that philosophy and science should at least operate under the assumptions of natural sciences (and thus physicalism). Physicalism is a strong form of metaphysical naturalism.

The ontology of physicalism ultimately includes whatever is described by physics — not just matter but also energy, space, time, physical forces, structure, physical processes, information, state, etc. Because it claims that only physical things exist, physicalism is generally a form of ontological monism.