  As we have seen, the Principle of Sufficient Reason is one of the bedrock principles of all of Leibniz's philosophy. In the Monadology, Leibniz appeals to PSR, saying that even in the case of contingent truths or truths of fact there must be a sufficient reason why they are so and not otherwise. (Monadology §36) But, since each particular truth of fact is contingent upon some other (prior) truth of fact, the reason for the entire series of truths must be located outside the series, and this ultimate reason is what we call God. (Monadology §37)