The Principle of Sufficient Reason in its classic form due to Leibniz states that every fact has a sufficient reason for why it is the way it is and not otherwise. The principle is at the heart of some versions of the cosmological argument for the existence of God. In fact, some philosophers, both theistic and non-theistic , think that the existence of God follows from the Principle.
The Principle can be seen to be a generalization of the dictum ex nihilo nihil fit, "nothing comes from nothing".