Kant’s most pressing criticism, in our view, goes back to the issue raised by Mersenne: we cannot determine whether God (conceived as having necessary existence or not) is really possible. Kant grants to Leibniz that the notion of a most perfect being may not involve a logical contradiction, but he argues that this is not enough to show that it is really possible, for there are ways of being impossible that do not involve logical contradictions (A602/B630). The implication for the ontological ar