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.