The Principle of Sufficient Reason (PSR) says that all contingent factsmust have explanations. In this volume, the first on the topic in the Englishlanguage in nearly half a century, Alexander Pruss examines the substantivephilosophical issues raised by the PSR, which currently is consideredprimarily within the context of various cosmological arguments for theexistence of God. Discussing several forms of the PSR and selected historicalepisodes from Parmenides, Aquinas, Leibniz, Hume, and Kant,Pruss defends the claim that every true contingent proposition must havean explanation against major objections, including Hume’s imaginabilityargument and Peter van Inwagen’s argument that the PSR entails modalfatalism. Pruss also provides a number of positive arguments for the PSR,based on considerations as different as the metaphysics of existence, counterfactualsand modality, negative explanations, and the everyday applicabilityof the PSR. Moreover, Pruss shows how the PSR would advancethe discussion in a number of disparate fields, such as metaethics and thephilosophy of mathematics.