Gassendi objects that existence is not part of the idea of God as a supremely perfect being. Can’t I form the idea of a God who does not exist? (This is similar to Hume’s objection.) Descartes replies by drawing claiming, with St Thomas Aquinas, that divine perfections all entail each other. Because our minds are finite, we normally think of the divine perfections – omnipotence, omniscience, necessary existence, etc. – separately and ‘hence may not immediately notice the necessity of their being joined together’. But if we reflect carefully, we shall discover that we cannot conceive any one of the other attributes while excluding necessary existence from it. For example, in order for God to be omnipotent, God must not depend on anything else, and so must not depend on anything else to exist.
However, Aquinas didn’t think that existence is a perfection. He objects, and Johannes Caterus put the point to Descartes, that the ontological argument doesn’t demonstrate that God really exists. It only shows that the concept of existence is inseparable from the concept of God. Descartes’ argument is only convincing for the claim that if God exists, God exists necessarily.
Descartes accepts that what he has shown is that necessary existence is part of the concept of God. But this, he responds, is enough: necessary existence entails actual existence. That God must exist to be God means that God exists. But Caterus says this isn’t enough: the question is whether the concept of necessary existence entails actual existence.