 Leibniz says that PSR is needed if we are to go beyond mathematics to metaphysics and natural science. How does the PSR help in those domains of inquiry? There is a general pattern of argument that Leibniz uses to establish conclusions using the PSR. First he assumes the falsity of what he wants to prove. Call the proposition to be proved p. Then he tries to show that if p were false, there would be some fact or truth for which there was no sufficient reason. But by the PSR, there is no fact or truth. Therefore, p is true. Leibniz uses this template to argue for a number of claims, including the identity of indiscernibles, relationalism with respect to space and time, and the existence of God. Let us briefly look at how Leibniz uses the PSR to argue for each of these theses.