Monads (from Greek monás, "unit"): infinitely small indivisible original components of the world made famous by the philosopher Leibniz.
In Greek philosophy a monad was something basic and indivisible — for Plato, an idea; for Democritus, an ►atom (to take two prominent examples). Leibniz developed a theory of monads to explain the structure of the world. According to that theory, all matter consists of infinitely many non-spatial, non-material monads. Hence, space and matter are infinitely divisible. Monads do not causally interact with one another since nothing external to a monad can affect it in any way; in Leibniz's famous metaphor, monads "have no windows". However, each monad "perceives" the entire ►universe "within itself", so to speak, as a result of its own activity. Hence, from this internal point of view, the monads also mediate forces between the sun, planets, and particles of matter.
Unlike the homogenous primordial matter ►Apeiron, monads are discrete individuals. Each differs from the others, since each "perceives" the universe from a different internal point of view. These individual perceptions are stored in memory and enable the monad to generate individual reactions. In contemporary terminology we might think of monads not as a type of elementary particle but rather as a kind of state machine that can pass from one inner state to the next. However, the difference is that state machines, as we know them, change states as a result of external impacts, while monads in Leibniz's theory are not supposed to be subject to any external influences once they have been created by God (which is why even God Himself cannot interfere with a monad once it is created). Due to their inherent faculties of perception and memory, monads also constitute ►consciousness and the souls of animals and humans.