Leibniz
Descartes had left two kinds of stuff, mind and matter, that could not touch. Spinoza had answered by melting everything down into one single substance. Leibniz looked at both answers and rejected them, and gave the strangest reply in the whole quarrel: reality is not one thing and not two kinds of thing but infinitely many things, simple soul-like points called monads, each one shut off from every other, each one privately mirroring the entire universe from where it sits. Nothing in this universe ever acts on anything else. The match between a person's decision and that person's arm rising, between any one thing and the rest, was arranged by God at the first instant and runs forever without contact, like a room full of clocks all keeping the same time though none of them is wired to any other. From that one picture he built a reason for why anything exists at all, a proof that no two things in the world are ever exactly alike, and the most hopeful and most mocked claim in philosophy: that this, with all its horror, is the best of all the worlds God could have made.
