Monadology

The Monadology is a short summary Gottfried Wilhelm Leibniz wrote near the end of his life, in 1714, of his metaphysical system. It is ninety numbered sections long. The bulk of the text is given to the doctrine of monads, the simple substances out of which Leibniz held the universe to be composed.

For this corpus the Monadology is the cleanest place to find Leibniz’s statement of the principle of sufficient reason and his version of the Cosmological Argument from contingency. Sections 31 through 39 contain the argument: nothing happens without a sufficient reason; the sufficient reason for the series of contingent things must lie outside that series; therefore there must exist a Necessary Being in which the sufficient reason of the series terminates.

The Monadology is the source most contemporary statements of the contingency argument cite, even when their direct ancestry is through later writers like Samuel Clarke or Pruss.