The Cartesian Circle?

[DirtDog]

So I have a philosophy test tomorow and there is going to be an essay style question asking me to explain the cartestian circle. I understand that it is a mistake in reasoning allegedly made by Rene Descartes. But I do not understand the mistake or the argument. Could anyone explain the meaning or help me understand this?

[moorobot]

Google it my man. And hope that the site is accurate.

[sigh]

[ QUOTE ]
"you are not yet certain of the existence of God, and you say that you are not certain of anything, and cannot know anything clearly and distinctly until you have achieved clear and certain knowledge of the existence of God. 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."

[/ QUOTE ]

-Marin Mersenne (The first to point out the flaw in Descartes' reasoning)