Monadology

A compressed summary of Leibniz's mature metaphysics in ninety numbered paragraphs, written in French in 1714 and left untitled; the title was given by later editors. It begins with monads, the simple substances out of which everything composite is made. Having no parts, monads cannot be created or destroyed naturally and have no windows through which anything could enter or leave. They differ only in their inner states, perceptions and the appetitions that move them from one perception to the next. In a well-known thought experiment, Leibniz imagines a thinking machine enlarged to the size of a mill: walking inside, we would find only parts pushing one another, never a perception. Two great principles govern our reasoning, contradiction and sufficient reason, and they lead to God as the necessary being who chose the best of all possible worlds. Each monad mirrors the whole universe from its own point of view, and soul and body agree through a harmony established in advance by God.

It is the shortest complete statement of one of the most ambitious systems in philosophy, and its mill argument is still cited in debates about whether machines could be conscious.