Materialism in philosophy, the view that all facts (including facts about the human mind and will and the course of human history) are causally dependent upon physical processes, or even reducible to them.
The many materialistic philosophies that have arisen from time to time may be said to maintain one or more of the following theses: (1) that what are called mental events are really certain complicated physical events, (2) that mental processes are entirely determined by physical processes (e.g., that "making up one's mind," while it is a real process that can be introspected, is caused by bodily processes, its apparent consequences following from the bodily causes), (3) that mental and physical processes are two aspects of what goes on in a substance at once mental and bodily (this thesis, whether called "materialistic" or not, is commonly opposed by those who oppose materialism), and (4) that thoughts and wishes influence an individual's life, but that the course of history is determined by the interaction of masses of people and masses of material things, in such a way as to be predictable without reference to the "higher" processes of thought and will.
Materialism is thus opposed to philosophical dualism or idealism and, in general, to belief in God, in disembodied spirits, in free will, or in certain kinds of introspective psychology. Materialistic views insist upon settling questions by reference to public observation and not to private intuitions. Since this is a maxim which scientists must profess within the limits of their special inquiries, it is natural that philosophies which attach the highest importance to science should lean toward materialism. But none of the great empiricists have been satisfied (at least for long) with systematic materialism.
The Greek atomists of the 5th century BC (Leucippus and Democritus) offered simple mechanical explanations of perception and thought--a view that was condemned by Socrates in the Phaedo. In the 17th century Thomas Hobbes and Pierre Gassendi, inspired by the Greek atomists, used materialistic arguments in defense of science against Aristotle and against the orthodox tradition, and in the next century the materialists of the Enlightenment (Julien de Lamettrie, Paul d'Holbach, and others) attempted to provide a detailed account of psychology.
During the modern period, the question of materialism came to be applied on the one hand to problems of method and interpretation in science (Henri Bergson, Samuel Alexander, A.N. Whitehead) and on the other hand to the interpretation of human history (G.W.F. Hegel, Auguste Comte, Karl Marx). Marx offered a new kind of materialism, dialectic and not mechanistic, and embracing all sciences.
In the 20th century, materialistic thought faced novel developments in the sciences and in philosophy. In physics, relativity and quantum theory modified, though they did not abandon, the notions of cause and of universal determinism. In psychology, J.B. Watson's behaviourism, an extreme form of materialism, did not find general acceptance; and researches both in psychology and in psychoanalysis made it impossible to hold any simple direct view of the mind's dependence on the processes and mechanisms of the nervous system. In philosophy, further reflection suggested to many that it is futile to try to erect a system of belief, whether materialistic or otherwise, on the basis of the concepts of science and of common sense (especially those of cause and of explanation).
Excerpt from the Encyclopedia Britannica without permission.