Descartes' Contemporaries
Many commentators, both at the time that Descartes wrote and since, have argued that this involves a circular argument, as he relies upon the principle of clarity and distinctness to argue for the existence of God, and then claims that God is the guarantor of his clear and distinct ideas. The first person to raise this criticism was Antoine Arnauld, in the "Fourth Set of Objections" to the Meditations:
"you are not yet certain of the existence of God, and you say that you are not certain of anything. It follows from this that you do not yet clearly and distinctly know that you are a thinking thing, since, on your own admission, that knowledge depends on the clear knowledge of an existing God; and this you have not proved in the passage where you draw the conclusion that you clearly know what you are." (AT VII 124–125)
Descartes' own response to this criticism, in his "Author's Replies to the Second Set of Objections", is first to give what has become known as the Memory response; he points out that in the fifth Meditation (at AT VII 69–70) he did not say that he needed God to guarantee the truth of his clear and distinct ideas, only to guarantee his memory:
"when I said that we can know nothing for certain until we are aware that 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." (AT VII 140)
Secondly, he explicitly denies that the cogito is an inference: "When someone says 'I am thinking, therefore I am, or I exist' he does not deduce existence from thought by means of a syllogism, but recognizes it as something self-evident by a simple intuition of the mind." (AT VII 140) Finally, he points out that the certainty of clear and distinct ideas does not depend upon God's guarantee (AT VII 145–146). The cogito in particular is self-verifying, indubitable, immune to the strongest doubt.
Read more about this topic: Cartesian Circle