This paper examines the Leibnizian background to Kant's critique of the ontological argument. I present Kant's claim that existence is not a real predicate, already formulated in his pre-critical essay of 1673, as a generalization of Leibniz's reasoning regarding the existence of created
things. The first section studies Leibniz's equivocations on the notion of existence and shows that he employs two distinct notions of existence – one for God and another for created substances. The second section examines Kant's position in his early paper of 1763. My claim
is that Kant's view of existence in 1763, namely that it is not a predicate, is strongly related to the logical notion of possibility, formulated by Leibniz and accepted by Kant.