THE ROLE OF GOD IN DESCARTES' SYSTEM
There is a paradox at the heart of Cartesian metaphysics. On the one hand, Descartes' whole system of scientific knowledge depends on our assured knowledge of God; but on the other hand, the idea of God is explicitly stated by Descartes to be beyond our comprehension. This paradox emerges in Descartes' proofs of God's existence, and hinges on the relationship between the affirmation of God's existence and the elucidation of the idea of God, which is the basis for that affirmation. The relationship is difficult to explicate precisely: is the idea of God prior to the demonstration of his existence?
All the proofs Descartes offers of God’s existence, whether a priori or a posteriori, make use of the idea of God. And we are told that "according to the laws of true logic, one must never ask if something exists [an sit] without knowing beforehand what it is [quid sit]" (AT VII 107-8: CSM II 78); in the absence of such prior knowledge, we could not identify as God the being whose existence we are demonstrating. The idea of God would thus appear to be a necessary premise for all the proofs of his existence, and this clearly implies that we must possess within us the relevant idea in order to be able to infer that its object or ideatum really exists outside our minds.