An ontological view, opposed to dualism, which holds that there is in reality only one sort of stuff in the universe, and thus that all existence can be reduced to this ultimate reality. In the context of the mind-body problem, it is the contention that there are no essential differences between the mental and physical domains, and which is sometimes referred to as materialism or physicalism. Two other broad categories of monism are idealism or phenomenalism (the opposite of materialism) and neutral monism (both the mental and physical can be reduced to some more ‘neutral’ or third kind of stuff and thus a sort of half-way house between dualism an monism). Identity theories of mind and body have grouped themselves into three types of monism: functionalism (the mental can be reduced to the physical, but something need not be made out of neurons to have mental states, a stance popularized in cognitive science and artificial intelligence), eliminativism (the same as radical behaviorism and thus a rejection of mental states), and anomalous monism (the priority of materialism over neutral monism and the view that while all mental things are physical, not all physical things are mental).
See Artificial intelligence (AI), Behaviorism, Cognitive science, Dualism, Folk wisdom, Mind-body problem, Ontology, Reductionism