  Leibniz made an important contribution to the history of the ontological argument. His reflections on this form of argument go back to the 1670s, and we know that he shared his thoughts on this matter with Spinoza when Leibniz visited him on the way to Hanover. According to Leibniz, the argument that Descartes gives implicitly in the Fifth Meditation and explicitly in the First Set of Replies is faulty. Descartes had argued that God is a being having all perfections, existence is a perfection, therefore, God exists. (AT VII 118–19/CSM II 84–85) But, Leibniz thinks, one needs to show that it is possible for such a being to exist, that is, that it is possible for all perfections to co-exist in one being. If this is so, then and only then an ens perfectissimum can be said to exist. In his short essay That a Most Perfect Being Exists (Quod ens perfectissimum existit) from 1676, Leibniz argues just this. He defines a “perfection” as a “simple quality which is positive and absolute, or, which expresses without any limits whatever it does express” (A VI iii 578/SR 101). And with this definition in hand, Leibniz is then able to claim that there can be no inconsistency among perfections, since a perfection, in being simple and positive, is unanalyzable and incapable of being enclosed by limits. That is, if A and B are perfections, then the proposition “A and B are incompatible” cannot be demonstrated because A and B are simples, nor can the proposition be known per se. Therefore, it is possible that any and all perfections are in fact compatible. And, therefore, Leibniz reasons, a subject of all perfections, or an ens perfectissimum, is indeed possible.