monism [Gr.,=belief in one], in metaphysics, term introduced in the 18th cent. by Christian von Wolff for any theory that explains all phenomena by one unifying principle or as manifestations of a single substance. Monistic theorists differ considerably in their choice of a basis of unification. It may be material, as with Ernst Haeckel, who took the substance, or energy, as the only reality. It may be spiritual, as with G. W. Hegel, to whom mind, or spirit, is the reality by which all is to be explained. Or, as in Spinoza, it may be a substance, or Deity, of which body and mind are attributes that are held in equipoise. The opposites of monism are dualism and pluralism.