Monism (from Greek mónos, “single”) — The metaphysical doctrine that reality reduces to a single principle, substance, or kind of thing, as opposed to dualism, which admits two, and pluralism, which admits many. The term is attributed to Christian Wolff, in the eighteenth century, who applied it to philosophers recognizing only one kind of substance, in contrast to Cartesian mind-body dualism; it became widespread in the nineteenth century. The idea itself is far older: in Parmenides, being is one, unmoving, and indivisible, and the Milesians sought a single principle (arché) of all things. The classic modern example is Spinoza, for whom there is a single substance, God or Nature, of which mind and body are attributes (Ethics, I and II), a position close to pantheism. Several forms are distinguished: materialist monism, which reduces everything to matter (Ernst Haeckel, The Riddle of the Universe, 1899); idealist monism, for which the real is spirit or idea (Hegel, Schelling, absolute idealism); and neutral monism, which holds that mind and matter are aspects of a reality neither mental nor physical in itself, defended by William James and Bertrand Russell (The Analysis of Mind, 1921). Opposed to it are Leibniz’s pluralism, whose monads are infinitely many individual substances, and William James’s pluralism (A Pluralistic Universe, 1909). See Spinoza, Parmenides, Leibniz, substance, materialism, and idealism.
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