Abstract: This work is an investigation into the ground of the principle of sufficient reason. My original contribution is the claim that in David Hume's writing on causality we find an implicit treatment of the principle of sufficient reason. While Hume does not explicitly accept or deny the principle of sufficient reason, my claim is that in analyzing causality, Hume also provides us with an account of the principle of sufficient reason, since causality may be understood as the empirical manifestation of the more general principle of sufficient reason. I support my claim about Hume by presenting two opposed views of the principle of sufficient reason, that of Leibniz and Schopenhauer. In my exegesis of Leibniz and Schopenhauer, I show how Leibniz's presentation treats the principle of sufficient reason as legitimately metaphysical, and Schopenhauer in his dissertation, On the Fourfold Root of the Principle of Sufficient Reason, treats the principle as a merely transcendental principle. These two polar views may be bridged, it is my claim, by looking to Hume's treatment of causality situated between them.