MonadologyGottfried Wilhelm Leibniz

The Monadology is ninety paragraphs long and contains an entire metaphysics. Leibniz wrote it in 1714, two years before his death, as a compressed summary of positions he had developed over decades.

The fundamental units of reality are monads: simple, indivisible, windowless substances. They have no parts, so they cannot be formed or destroyed by natural means. Each monad perceives the entire universe from its own point of view, with varying degrees of clarity. A stone's monads perceive dimly; an animal's more distinctly; a rational soul perceives with apperception, conscious awareness of its own states.

Monads do not interact. What appears to be causal influence between bodies is in fact a pre-established harmony: God has arranged all monads from the beginning so that their internal states correspond to one another perfectly, like clocks synchronized by a master clockmaker. There is no real space, no real extension; space is the order of coexisting phenomena, and matter is the confused appearance of monadic activity.

The principle of sufficient reason governs everything. Nothing exists without a reason, and the reason the actual world exists rather than any other possible world is that God chose the best. This is the doctrine Voltaire mocked in Candide, but Leibniz's argument is more subtle than the caricature suggests: "best" means the maximum of variety with the maximum of order, not the absence of suffering.