In philosophy of mind and cognitive science, materialism refers to the belief that the mind is the product of the material world — that is, that the mind is created by the brain, and that phenomena like consciousness exist by virtue of physical laws operating upon matter. It contrasts with dualism, which describes the belief that the body and the mind, or soul, are fundamentally different creatures. Religions that preach an afterlife or reincarnation implicitly endorse dualism, as they believe that the soul survives the destruction of the body.
Materialism is in essence an attempt to solve the mind-body problem: how is it that an event in the mind can cause a change in the body, if the two are unrelated? If I decide to move part of my body, how does that decision actually come to influence my body? And in reverse, if something happens to my body, how does my mind become aware of it? Materialism answers that by conceptualizing mental states and events as nothing more than particular kinds of physical events; my decision to move my body is just a particular brain process. And if I burn myself on the stove, the subjective feeling of pain is nothing but a particular pattern of excitation inside my brain.
Further, materialism answers the obvious question of how it can be that the mind depends upon the matter of the brain for its functioning. If the brain is damaged, or if chemicals alter its functioning, it's clear that it has impacts upon the mind; the dualist must answer how it is that the mind may be altered by physical phenomena, or even rendered permanently unconscious while the body remains alive in the case of a coma.
The origins of materialism and dualism
The oldest known discussion of materialism comes from De Rerum Natura, On The Nature of Things, a poem by Roman philosopher Lucretius. Lucretius stated in the poem that belief in gods and fear of death were superstition, because the mind was annihilated at death. He believed that all phenomena were based upon the interactions of atoms and offered naturalistic explanations for numerous physical phenomena.
Dualism underlay the thoughts of most ancient Greek philosophers; Plato and Aristotle endorsed the notion that the mind was not subject to physical law. But the figure most identified as foundational to modern dualism was René Descartes, reasoning that while he could wonder whether his body was an illusion, the act of wondering itself proved that the mind was not. He believed that self-awareness was a property of the mind, though intelligence belonged to the brain. Thus he was first to explore the mind-body problem, one he couldn't solve. Some of his disciples proposed that God mediated the actions of the mind and body, and thus they proposed a strictly religious solution to the problem.
Varieties of materialism
Several flavors of materialism exist. Reductive materialism holds that conscious states — sensory perceptions, emotions, and knowledge — correspond to physical states in the brain. If you see something, it's because particular neurons are activated, and those activations lead to your awareness of it. Further, according to reductive materialism, the interactions in the brain that lead to mental states can be explained as special cases of more general physical rules. Nonreductive materialism, in contrast, claims that even empirical laws in many "special" sciences — psychology, geology, and so on — can't be usefully explained as instances of more general physical laws. Jerry Fodor, a philosopher at Rutgers University, is one of the most important figures arguing against reductionism in cognitive science and philosophy of mind. Eliminative materialism goes further than either of the previous two forms in rejecting the special nature of thought, and claims that most of our belief in mental states is baseless, and that even phenomena such as emotions, desires, and consciousness itself are illusions stemming from folk psychology theories about how the mind works that have no basis in reality.
One particular type of materialism that has major support among many cognitive scientists is emergent materialism. Emergence studies the phenomenon that many complex systems can be composed of many parts interacting according to simple rules but creating a complex behavior that emerges when the whole system is examined. Ant colonies are a frequently cited example of emergence; while individual ants operate along simple procedures — following and leaving chemical trails — the colony as a whole seems to know things and engage in behaviors as a system that none of the individual ants do. Ant colonies, for instance, quickly find the most efficient routes to nearby sources of food and coordinate the building of 'graveyards' of dead ants, even though there is no evidence that individual worker ants have any overall understanding of these operations. According to emergent materialism, a biological system as complex as a human brain can be composed of simple parts — that is, neurons — interacting along simple rules, but together creating extremely complex behaviors. One of this view's chief proponents is John Searle, whose formulation of Searle's Chinese Room was in part to promote his view that only the complexity of a brain or something very like it, rather than a simple set of instructions executed by a digital computer, could explain phenomena like consciousness.
Materialism in cognitive science
Artificial intelligence is an important part of cognitive science; much of the initial thinking underlying the formation of the discipline of cognitive science came from early efforts to develop artificial intelligence in the primitive computers of the mid-twentieth century. It continues to play a strong role in evaluating theories about how cognition works, by providing the best ground we know of to test hypotheses. A theory of how human cognition works can be tested by implementing it in a computer program and examining whether the computer's output resembles that of humans. Artificial intelligence essentially presupposes materialism, since unless the mind is some aspect of the brain's functioning, attempting to model or duplicate it in silico seems fruitless.
It may seem that cognitive science itself presupposes materialism, because it makes little sense to try to scientifically investigate what is outside the physical world. Certainly, scientific research trying to determine the origins of consciousness is not easily compatible with dualism, although some areas of cognitive science — determining how people perform particular cognitive activities — are somewhat compatible with the notion that the mind is somehow outside the body, as long as it can be investigated by examining its input from and output to the physical world.
References
Thagard, Paul, 2005. Mind: Introduction to Cognitive Science. MIT Press.
Place, U.T., 1956. "Is Consciousness a Brain Process?". British Journal of Psychology.
Churchland, Paul M., 1981. "Eliminative Materialism and the Propositional Attitudes". The Journal of Philosophy.