 A third sort of a priori proof argues from facts about the mere possibility of something (or of some collection of things) to the existence of a “ground of possibility” that somehow explains them. An early version of such an argument can be found in Augustine and the Neo-Platonic tradition, but the canonical presentations of this sort of “possibility proof” are in Leibniz’s Monadology (1714) and, much more elaborately, in Kant’s book-length treatise called The Only Possible Basis for a Demonstration of the Existence of God (1763). Note that if truths about what is possible are necessary truths, as they indeed seem to be, then this might be a more specific version of the argument from necessary truths (see  2.2).