Leibniz on the PSR as a Regulative Principle of Rational Inquiry

One of Leibniz’s fundamental philosophical commitments is the principle of sufficient reason (PSR). Yet Leibniz’s precise understanding of this principle is elusive. Leibniz provides several seemingly different formulations of its content, and Leibniz sometimes seems to require the PSR being metaphysically necessary and sometimes being metaphysically contingent. I argue that these puzzles can be solved by taking seriously Leibniz’s insistence that the PSR is a principle of reasoning. For Leibniz, I argue, to say that the PSR is a principle of reasoning is to say that it is a regulative principle of rational inquiry, a principle that settles some of the permissible and impermissible moves in the game of rationally inquiring into a fundamentally intelligible reality. According to this reading, there is a coherence between epistemology and metaphysics: for Leibniz, one is epistemologically justified in holding metaphysical doctrines precisely because they result from engaging in the type of rational inquiry structured by the PSR understood as a principle of reasoning.