From early on in Leibniz's philosophical career until his very last letter to Clarke, Leibniz was keen to emphasize the importance of what he called his “two great principles.” In the familiar words of the Monadology, “all our reasonings are based on two great principles, that of contradiction … and that of sufficient reason.” If the first of these is presupposed by his logic generally and (as he implies in the Monadology and the Clarke Correspondence) by all reasoning about logical possibility and necessity in particular, it is undoubtedly the second of the two – the Principle of Sufficient Reason (PSR) – that figures most prominently in Leibniz's metaphysical theorizing. Indeed one can think of scarcely any central doctrine in Leibniz's metaphysics that is not beholden in some fairly direct way to the Principle of Sufficient Reason. Leibniz explicitly reckons PSR a necessary condition for his concept-containment account of truth (C 518–19: L 267–68), and thereby (in Leibniz's hands, for good or ill) in the doctrines of spontaneity and marks and traces (G IV,433: L 307–08); PSR is a crucial premise in several of Leibniz's arguments for the existence of God, in his account of volitional actions by creatures and by God, in his argument that ours is the best possible world, and in his relationalist arguments against absolute space and time.