[From Latin dualis: containing two.]
(metaphysics) A doctrine in metaphysics or cosmology positing that there are only two fundamental things, substances, or aspects of reality in the universe at large or in human psychology. The first influential dualist theory in Western philosophy was Platonism, since Plato claimed that there are two different realities: the physical world of appearances and the higher world of intelligible forms, ideas, or essences, with a similar separation in the human person between mind and body. These ideas were assimilated by Stoicism and, later, by Christianity. While dualism was influential throughout the Christian era, it received renewed impetus from Descartes, who held that reality is made up exclusively of spirit and matter, and that these two substances can never meet or interact -- except in the human soul (which gives rise to the modern mind-body dichotomy). Aristotelianism, by contrast, generally holds that mind and body are not two distinct substances but two aspects of a complete human person (cf. holism). Even though dualism is a kind of metaphysical pluralism and is opposed by monism, practically speaking dualists often emphasize the 'higher', more spiritual reality, so that they are often construed as adherents of idealism or transcendentalism, even though this is not strictly the case.