| Many people have argued that Descartes' philosophy contains a vicious circularity. His circular reasoning revolves around his ideas of God and clear and distinct perceptions:
1) Descartes states that whatever he clearly and distinctly perceives is true. In other words, if an idea is clear and comprehended, and distinct and not mixed up with any others, it must be true.
2) He then uses his clear and distinct ideas to prove God's existence (in his cosmological argument, and his ontological argument).
3) Descartes then uses his idea of God to show that his clear and distinct perceptions are true (as part of Descartes on truth, falsity, and the faculties of the mind).
Here the argument looks thoroughly circular. It is not a logically sound strategy to use one's conclusion to support and affirm one's premises - this is "begging the question". However, have we misinterpreted what Descartes was saying? Was his argument really as formulated above? Descartes says no;
"When I said that we can know nothing for certain until we are aware God exists, I expressly declared that I was speaking only of knowledge of those conclusions which can be recalled when we are no longer attending to the arguments by means of which we deduced them."
This is known as the "memory response". So Descartes is saying that he is not using God to guarantee that his clear and distinct perceptions are true, but rather his accuracy of their recollection. Does this free Descartes from the circularity criticism? There is now a new problem associated with how Descartes shows that God exists. Descartes proofs of God are deductive arguments. As such they are in the form of a series of premises, followed by a conclusion. Let us suppose, despite the thought to the contrary, that all of Descartes premises are true. These premises are, according to Descartes, arrived at by clear and distinct perceptions, so there is no problem with him knowing that they are true. However, the problem comes as he uses them to prove God's existence. Without having yet demonstrated that God exists, Descartes cannot be certain that he is recollecting his premises correctly. He therefore cannot know his conclusion is based on accurate argument. As it stands, we still have circularity - it has just shifted to a slightly different aspect of his proof of God.
Besides, there is another problem. Descartes does not really support his ultimate premise that clear and distinct perceptions are true. He seems to have derived the premise that clear and distinct perceptions are true from the fact that he clearly and distinctly perceives it. This is not entirely convincing.
However - this touches on a deep issue of philosophy associated with how we can reason that our methods of reason are valid. | 
