The Principle of Sufficient Reason (PSR) holds that everything has an explanation. My dissertation defends the PSR—a principle that many take to be a prime tenet of the rationalist metaphysics of Leibniz and Spinoza—from three influential challenges: (1) that we lack reason to accept the PSR; (2) that the PSR entails that the world could not have been otherwise; (3) that the principle is inconsistent with the now widespread recognition of metaphysically fundamental facts. By answering these challenges, I resist the contemporary dismissal of this central rationalist tenet. To endorse the PSR requires rejecting any view that admits unexplained or ‘brute’ facts. But such views are pervasive in contemporary metaphysics. My dissertation therefor...