 The challenge in the examination of the idea of God is to account for the origin of the idea’s level of objective reality. He determines that the formal reality possessed by his own mind cannot be its origin. He concludes that there must be some being that in fact possesses the requisite level of formal reality, which in this case will be greater than that of a finite substance. (For more, see the SEP entry on  Descartes’ epistemology.)  Notice how this differs from what he says about the formal reality of an idea, namely, that his mind is the cause or origin of an idea’s formal reality.