Dualism is a philosophical concept that refers to the idea that there are two fundamental kinds or categories of things or principles. In the philosophy of mind, dualism is the theory that the mental and the physical, or mind and body, are in some sense, radically different kinds of things. Dualism is closely associated with the thought of René Descartes, who held that the mind is a nonphysical substance and therefore non-spatial. There are different types of dualism, including substance dualism, which asserts that mind and matter are fundamentally distinct kinds of foundations. Property dualism, on the other hand, may be considered a form of emergent materialism or non-reductive physicalism in some sense.

Dualism is contrasted with various kinds of monism, which is the theory that there is only one fundamental kind, category of thing, or principle. Dualism is also distinguished from pluralism, which is the view that there are many kinds or categories.

Opponents of dualism argue that it is inconsistent with known laws or truths of science, conceptually incoherent, or reducible to absurdity. Despite criticisms, dualism remains an important concept in philosophy and has been used to analyze the knowing process or to explain all of reality or some broad aspect of it.