But how could one know that the true and immutable nature of a being is to exist purely on the basis of an idea? In fact, this argument works better for a triangle, since its essence is implied in the definition. It does not hold, however, for God, whose definition is contestable. Descartes would resort to saying that any other concept of God would be self-contradictory. This is not necessarily true. One's definition of God's attributes does not make it the case.
Descartes recognizes the major objection to this position: that his thinking the idea of God does not make God exist. What a person thinks does not create the thing. He says, "For my thought does not impose any necessity on things; and just as I may imagine a winged horse even though no horse has wings, so I may be able to attach existence to God even though no God exists" (VII: 66). But his response is flawed. He flips the priority around: it is not that his thinking determines God's existence, but that God's existence necessarily determines his thinking. He is "not free to think of God without existence" (VII: 67), even though he could with a winged horse. This argument presupposes the existence of a perfect God, and does not imply it. It is not the result of his argument, but its assumption. To believe that one's thinking is determined by the thing in reality is to assume its prior existence, not to prove it before existence. All it does is replicate the truth of his idea. It proves that he has a particular idea of the essence of God, not that a perfect God with existence exists outside his mind. His flaw is...
Here another important point arises, which he does not consider: one might never actually have an idea of God. Descartes proof for God's existence relies on necessarily having an idea of God. But what if one never had the idea in the first place? Or if one's idea of God was not clear, but indistinct? Descartes ontological proof does not address this issue. It merely presumes that any natural person will necessarily have an idea of God.
In sum, Descartes assumes realism, but it remains a presupposition not a conclusion. He does not prove psychologism wrong, or realism right. It is better to understand that God is eternally improvable as to real existence. Clearly God is a possible idea, but the idea alone does not make the reality. Nor does he prove that one must ever have an idea of God. All Descartes has succeeded in proving is that a concept of God is possible, not that it is necessary. God may yet be only a fiction, invented by the mind, even if there are real effects and emotions attached to this object. Descartes ontological argument is best abandoned for a more psychological understanding of the possibility of the idea of God that need not imply God's actual existence.
Bibliography
Descartes, Rene. Meditations on First Philosophy with Selections from the Objections and Replies. Trans. John Cottingham. Cambridge: Cambridge University Press, 1996.
