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