The monads: true atoms of nature

Descartes made the essence of body extension (res extensa): to be a physical substance is to be spread out in space. Leibniz thinks this cannot be the ground floor of reality, because extension can never yield a genuine unity — and without unities, there is nothing real for the world to be made of.

Here is the argument. Whatever is extended is thereby infinitely divisible: any stretch of space has a left half and a right half, each divisible again without end. So an extended thing is always a plurality — an aggregate of parts, and those parts of further parts, all the way down. It is a heap, a crowd, a many. It is never one thing in its own right.

So Leibniz lays down a principle that reads almost like a slogan: “what is not truly one being is not truly one being” (letter to Arnauld, 1687). Reality — being — comes in units. If nothing is a true unity, nothing is truly real. Therefore the ultimate constituents of reality cannot be extended, divisible aggregates. They must be simple: without parts.

These simple substances are the monads (from Greek monas, “unit”). Leibniz calls them the “true atoms of nature” and “the elements of things” (Monadology §1–3). Being partless, a monad is unextended — it takes up no space; it is a metaphysical point, not a physical speck. And being partless, it can neither be assembled nor taken apart: monads can come to be only by creation and end only by annihilation (§4–6).

Because a monad has no parts and no extension, nothing can get into it or out of it — there is no interior to rearrange from outside. Leibniz’s famous image: monads are “windowless” (Monadology §7). “The monads have no windows through which anything could enter or leave.” No monad is causally acted on by another; all its changes well up from within, “from an internal principle,” following from its own nature.

But if a monad has no parts, what does it do — how does it change at all? Leibniz gives every monad two inner features. First, perception: an internal state that represents, or “mirrors,” the rest of the universe from that monad’s particular point of view (Monadology §14). Every monad is “a perpetual living mirror of the universe” (§56) — a whole cosmos folded up inside a point.

Second, appetition (appetite): the internal principle of change — a striving that carries the monad from one perception to the next (Monadology §15). Perception gives the monad content; appetition gives it a built-in tendency to keep unfolding to new content. Together they are the entire inner life of a monad; a monad is, in effect, a momentary mind in constant self-driven flux.

Monads therefore differ from one another only in the clarity and distinctness of their perceptions. This yields a continuous hierarchy of being: at the bottom, “bare” monads (monades nues) with only dim, confused perception (the stuff of rocks and dust); higher up, souls with distinct perception, memory and feeling (animals); higher still, rational souls or spirits capable of apperception, reason, and grasping necessary truths (human minds, Monadology §29); and at the summit God, the supreme Monad, whose perception of the universe is perfectly clear and distinct.