MONAD. — The term, derived from the Greek μονάς (unity), was already used by Pythagoras and his disciples who, with their theory of number as the essence of things, made the monad the ultimate element of reality (Philolaus: ἡ μὲν μονὰς ὡς ἂν ἀρχὴ οὐσία πάντα). Diels, *Vorsokrat.*, 32 B 8). Subsequently, it has often been used to denote either the indivisible particle of things, as a synonym for the more common term atom, or the principle of unity, such as Plato’s Ideas and God.
However, the term is chiefly remembered for its use by Leibniz (q.v.). In the Leibnizian system, monads are unextended, simple, spiritual substances, endowed with perception and appetition, closed in upon themselves «without doors or windows», interconnected by pre-established harmony, each reflecting the entire universe as in a living mirror, and arranged in a continuous gradation from the lowest, which constitute bodies, through the soul and mind, up to God, the primitive monad.