Is Descartes' argument for the existence of God circular?

In his Meditations on First Philosophy, René Descartes actually offers three distinct proofs of God's existence. While none of them are circular, they do posit claims that Descartes thinks self evident but are not universally acknowledged as either self-evident or true by many other philosophers.

First, Descartes proffers what is called the ontological argument for the existence of God. This starts with the notion that we conceive of God as perfect. Since existence is...

In his Meditations on First Philosophy, René Descartes actually offers three distinct proofs of God's existence. While none of them are circular, they do posit claims that Descartes thinks self evident but are not universally acknowledged as either self-evident or true by many other philosophers.

First, Descartes proffers what is called the ontological argument for the existence of God. This starts with the notion that we conceive of God as perfect. Since existence is more perfect than non-existence, our conception of God must include a conception of his existence. While this is not circular per se, it is problematic in treating existence as a quality, assuming that something we can conceive must exist, ands assuming that existence is necessarily more perfect than non-existence (some modern theologians, in fact, argues that the perfection of God lies precisely in the way God differs from existing things).

Descartes' other two arguments rely on the notion of causation. First, for me to exist as a being with ideas about God, there must be some cause to that existence. That cause must be external to me, prior to me, and greater than me. Thus we need to posit God as a cause to account for our existing as beings with concepts of God. 

In both ontological and causal arguments for God, the problem is not circularity so much as confusing epistemology (our knowledge of something) with ontology (the existence of that thing). We can imagine Pegasus, or Zeus, or Harry Potter, but most of us would claim that those things do not "exist" outside our own imaginations. The problem is far from intractable, as either we can start refining our concept of existence to make it encompass all possible objects of thought or we can argue for God as a necessary ground for knowledge, but neither of these are precisely the arguments set forth by Descartes.