Leibniz, and many following him, saw the Principle of Sufficient Reason (PSR) as pivotal to a scientific (demonstrated) metaphysics. Against this backdrop, Kant is expected to pay close attention to PSR in his reflections on the possibility of metaphysics, which is his chief concern in the Critique of Pure Reason. It is far from clear, however, what has become of PSR in the Critique. On one reading, Kant has simply turned it into the causal principle of the Second Analogy. On a different reading, PSR is but the supreme principle of reason, which roughly states that, if the conditioned is given, so is the unconditioned. On my reading, PSR appears in the guises of both the causal principle and the supreme principle. This twofold specification, I argue, is key to understanding (i) Kant’s allegations that past metaphysicians failed to prove PSR, (ii) his own Critical account of the possibility of metaphysics in both of its parts (ontology and metaphysics proper), and (iii) his nuanced answer to the contentious question about the relation between physical inquiry and metaphysical reasoning about nature (both being quests for reasons).