Principle usually associated with Leibniz, for whom it had a fundamental status, although found in earlier medieval thought, particularly that of Abelard. It is sometimes described as the principle that nothing can be so without there being a reason why it is so. But the reason has to be of a particularly potent kind: eventually it has to ground contingent facts in necessities, and in particular in the reason an omnipotent and perfect being would have for actualizing one possibility rather than another. Among the consequences of the principle is Leibniz's relational doctrine of space, since if space were an infinite box there could be no reason for the world to be at one point in it rather than another, and God placing it at any one point would violate the principle. In Abelard, as in Leibniz, the principle eventually forces the recognition that the actual world is the best of all possible worlds, since anything else would be inconsistent with the creative power that actualizes possibilities.