A philosophical concept first introduced by Leibnitz, the polymath philosopher, although it has precursors in almost every major thinker before him. A possible world in Leibnitz's sense is a way the entire world could have been; that this world, the actual world is the way it is is because God actualized it. (Due to his theological commitments, he thought that the actual world was the best of all possible worlds.)

In modern philosophy the concept has been made for the most part less metaphysical and more logical. Saul Kripke and Alvin Plantinga consider a possible world to be a maximal consistent set of propositions. On the other hand, a modal realist such as David K. Lewis keeps a metaphysical notion; a possible world to him is a world, though not necessarily the actual one. Lewis has argued at length for the metaphysical existence of possible worlds.