  The Principle of Sufficient Reason (PSR) in its classic form is simply that nothing is without a reason (nihil est sine ratione) or there is no effect without a cause. As Leibniz remarks, this principle “must be considered one of the greatest and most fruitful of all human knowledge, for upon it is built a great part of metaphysics, physics, and moral science” (G VII 301/L 227). In the Principles of Nature and Grace, Leibniz suggests that the claim that nothing takes place without a sufficient reason means that nothing happens in such a way that it is impossible for someone with enough information to give a reason why it is so and not otherwise. In the Monadology and elsewhere, however, Leibniz frankly admits that “most of the time these reasons cannot be known to us” (G VI 612/AG 217). While the idea that every event must have a cause and that there is a reason why everything is so and not otherwise again might not seem novel, it is the connection that Leibniz sees between this principle and his other metaphysical principles that is noteworthy. According to Leibniz, PSR must actually follow from PIN, for if there were a truth that had no reason, then there would be a proposition whose subject did not contain the predicate, which is a violation of Leibniz's conception of truth.