Physicalism, of which materialism is a historical antecedent, is primarily an ontological doctrine concerning the nature of reality and, specifically, mental reality. It is the view that reality is ultimately constituted or determined by entities—objects, events, properties, and so on—that are physical. This thesis is often combined with a claim about the explanatory supremacy of physical theory (physics).

Any formulation of physicalism raises the question, What is meant by "physical"? It is difficult to formulate a conception of the physical that is neither too strong, making physicalism obviously false, nor too weak, making physicalism trivially true. For example, what is physical may be simply identified through the language of physics. However, a problem arises over the conception of physics appealed to. Current physics seems too narrow because future extensions of physics would not count as physical; but the idea of a completed physics is too indeterminate...