Materialism, in philosophy, doctrine that all existence is resolvable into matter or into an attribute or effect of matter. According to this doctrine, matter is the ultimate reality, and the phenomenon of consciousness is explained by physiochemical changes in the nervous system. Materialism is thus the antithesis of idealism, in which the supremacy of mind is affirmed and matter is characterized as an aspect or objectification of mind. Extreme or absolute materialism is known as materialistic monism. According to the mind-stuff theory of monism, as expounded by the British metaphysician W. K. Clifford, in his Elements of Dynamic (1879-87), matter and mind are consubstantial, each being merely an aspect of the other. Philosophical materialism is ancient and has had numerous formulations. The early Greek philosophers subscribed to a variant of materialism known as hylozoism, according to which matter and life are identical. Related to hylozoism is the doctrine of hylotheism, in which matter is held to be divine, or the existence of God is disavowed apart from matter. Cosmological materialism is a term used to characterize a materialistic interpretation of the universe.

Antireligious materialism is motivated by a spirit of hostility toward the theological dogmas of organized religion, particularly those of Christianity. Notable among the exponents of antireligious materialism were the 18th-century French philosophers Denis Diderot, Paul Henri d'Holbach, and Julien Offroy de La Mettrie. According to historical materialism, as set forth in the writings of Karl Marx, Friedrich Engels, and Vladimir Ilich Lenin, in every historical epoch the prevailing economic system by which the necessities of life are produced determines the form of societal organization and the political, religious, ethical, intellectual, and artistic history of the epoch.

In modern times philosophical materialism has been largely influenced by the doctrine of evolution and may indeed be said to have been assimilated in the wider theory of evolution. Supporters of the theory of evolution go beyond the mere antitheism or atheism of materialism and seek positively to show how the diversities and differences in creation are the result of natural as opposed to supernatural processes.