  In the course of the Third Meditation, Descartes constructs an argument for the existence of God that starts from the fact that he has an idea of an infinite being. The argument is intricate. It invokes the metaphysical principle that “there must be at least  as much reality in the efficient and total cause as in the effect  of that cause” (7:40). This principle is  put forward as something that is “manifest by the natural light” (7:40), which  itself is described as a cognitive power whose results are indubitable (7:38), like clear and distinct perception (7:144). Descartes then applies that principle not to the mere existence of the idea of God as a state of mind, but to the content of that idea. Descartes characterizes that content as infinite, and he then argues that a content that represents infinity requires an infinite being as its cause. He concludes, therefore, that an infinite being, or God, must exist. He then equates an infinite being with a perfect being and asks whether a perfect being could be a deceiver. He concludes: “It is clear enough from this that  he cannot be a deceiver, since it is manifest by the natural light that all fraud and deception depend on some defect” (7:52).