Abstract: Many interpreters have supposed that the root of contingency in Leibniz's thought is that it is contingent rather than necessary that God chooses to create the best possible world. It is far from clear, however, that Leibniz believed this. This chapter argues that Leibniz did believe two theories of contingency: one based on the notion of a thing's being possible in itself whether or not a perfectly wise and good God could choose it, and one based on an identification of necessity with provability, where an infinite analysis does not count as a proof. It emerges that Leibniz's conception of possible worlds is not exactly the same as that now used in possible worlds semantics.

Keywords: best possible world, contingency, God, Leibniz, necessity, possibility, possible worlds,