 The very concept of a “nonexistent object” has an air of paradox about it, at least for those philosophers whose thinking is rooted in the Humean tradition. For Hume suggested that to think of an object is always and necessarily to think of an existent object, or to put it differently, that to think of an object and to think of the same object as existing are just one and the same thing. Immanuel Kant took up Hume’s idea and claimed that existence is not a “real predicate”, a claim that is often interpreted as an anticipation of Gottlob Frege’s famous doctrine that existence is not a predicate of individuals. (See Hume 2000, Book 1, Part 2, Sect. 6; Kant 2003, B 627; Frege 1966, pp. 37f.) Kant’s motivation for rejecting the view that existence is a “real predicate” was the so-called “ontological proof” of God’s existence, which says, roughly, that God’s perfection entails God’s existence, since a being that would have all of God’s perfections except existence (i.e., omniscience, omnipotence, benevolence) would be less perfect than a being with the same perfections plus existence. For centuries, philosophers have felt that there is something wrong with this proof, but Kant was the first one who was able to point out a possible error: he argued that the mistake of the “ontological argument” lies in the treatment of existence as a “real predicate”.