dualism, any philosophical system that seeks to explain all phenomena in terms of two distinct and irreducible principles. It is opposed to monism and pluralism. In Plato's philosophy there is an ultimate dualism of being and becoming, of ideas and matter. Aristotle criticized Plato's doctrine of the transcendence of ideas, but he was unable to escape the dualism of form and matter, and in modern metaphysics this dualism has been a persistent concept. In modern philosophy dualism takes many forms. Thus in Immanuel Kant there is an ontological dualism between the phenomenal and noumenal worlds and an epistemological dualism between the passivity of sensation and the spontaneity of the understanding. In psychology occasionalism and interactionism both assumed a dualism of mind and matter. The term also has a theological application, e.g., Manichaeism explained evil in the world as resulting from an ultimate evil principle, coeternal with good. See also monism and pluralism.

Metaphysical problem of the relationship between mind and body. The modern problem stems from the thought of René Descartes, who is responsible for the classical formulation of dualism. Descartes's interactionism had many critics even in his own day. Thomas Hobbes denied the existence of mental substance. Materialism of a sort was also supported by Descartes's correspondent Pierre Gassendi (1592–1655). Benedict de Spinoza posited a single substance of which the mental and the material are attributes; his theory is known as psycho-physical parallelism. More recent views include the double-aspect theory, identity theory, eliminative materialism (which denies the reality of the familiar categories of mental state posited in so-called folk psychology), and theories of supervenience.

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In philosophy, any pair of irreducible, mutually heterogeneous principles used to analyze the nature and origins of knowledge (epistemological dualism) or to explain all of reality or some broad aspect of it (metaphysical dualism); also, any theory that employs dualisms. Examples of epistemological dualisms are subject and object and sensation and sensibilia; examples of metaphysical dualisms are mind and matter, good and evil, and God and world. Dualism is distinguished from monism and pluralism.

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