APA Dictionary of Psychology

n. in the thought of Gottfried Wilhelm Leibniz, one of the ultimate indivisible units of existence. Monads are independent of one another and innately have the power of action and direction toward some end (see nisus). Although no monad in reality acts on any other, they work in a divinely preestablished harmony so that an appearance of causal connection is maintained. The concept of the monad was intended, in part, to address the mind–body problem arising from Cartesian dualism.