Dualism — A metaphysical position holding that reality is grounded in two principles, substances, or categories that are irreducible to one another—that is, that cannot be reduced to or explained in terms of a single foundation. The term comes from the Latin dualis (from duo, “two”) and stands opposed to monism, which recognizes a single fundamental principle, and to pluralism. Its most influential formulation is René Descartes’s substance dualism, set out above all in the Meditations on First Philosophy (1641): reality divides into res cogitans, the thinking substance (the soul or mind, without spatial extension), and res extensa, the extended substance (the body and the material world). From this arises the so-called mind-body problem: if mind and body are radically distinct substances, how can they interact, so that a decision of the will moves the arm, or a physical injury produces pain? Descartes went so far as to suggest that this interaction took place in the pineal gland, a solution his critics found unsatisfactory. Cartesian dualism had precursors: Plato already distinguished the immortal soul from the perishable body. In contemporary philosophy of mind, substance dualism is distinguished from property dualism, which admits a single substance endowed with irreducible mental and physical properties. The dualist scheme reappears in other domains: good and evil in Manichaeism, phenomenon and thing-in-itself in Kant, the sacred and the profane in religion. It is opposed both by materialism and by idealism, which seek to reduce everything to a single kind of reality.
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