Materialism holds that only physical matter exists. Mind, consciousness, and spirit are either identical to physical states or reducible to them. The view has ancient roots. Democritus and Epicurus argued that the universe consists of atoms and void. Hobbes and La Mettrie revived materialism in the modern period. In the 20th century, it became the dominant position in philosophy of mind and cognitive science, though the hard problem of consciousness remains a challenge.

Materialists come in several varieties. Reductive materialists hold that mental states are identical to brain states. Eliminative materialists argue that folk psychology, with its talk of beliefs and desires, will be replaced by neuroscience. Non-reductive materialists accept that mental states are physical but deny that they can be reduced to physics. Functionalism holds that mental states are defined by their causal roles, not by their physical substrate. Materialism is not the same as behaviorism, which denies the existence of inner mental states altogether. It is also not the same as physicalism, though the terms are often used interchangeably. Materialism is a metaphysical thesis about what exists. Physicalism is often broader, including physical properties and laws.

Forms of materialism

• Reductive. Mental states are brain states.
• Eliminative. Folk psychology will be replaced.
• Non-reductive. Mental states are physical but irreducible.
• Functionalist. Mental states are causal roles.

Materialism faces the hard problem: why does physical processing give rise to subjective experience? No one has a fully satisfying answer.