Monad
Leibniz's term for the simple, immaterial, individual substances that constitute reality — windowless atoms of perception that mirror the universe from a unique perspective.
The monad is the central concept of Gottfried Wilhelm Leibniz's mature metaphysics, set out in the Monadology (1714). Leibniz holds that what fundamentally exists is not extended material substance but a plurality of simple, immaterial, indivisible substances — the monads — each of which is a unique perspective on the universe.
Monads are windowless: nothing comes into them from outside, and they have no real causal interaction with one another. Yet they are coordinated by a divinely instituted pre-established harmony, so that each monad's internal sequence of perceptions corresponds to what would have been the joint development of an interacting system. Bodies and the apparent world of extension are well-founded phenomena — orderly appearances grounded in the underlying community of monads. The monadology is one of the most ambitious metaphysical systems in early-modern philosophy and shaped subsequent debates about substance, perception, and the relation of mind to world.