Mithus, here is a link http://www.philosophypages.com/hy/4b.htm that pretty succinctly explains Descartes in relation to the matter and also provides a bit of context.

Sinij more or less hit the nail on the head earlier in explaining your use of circular logic. He tends to put things differently, but he was substantively correct. I do find that circular logic is not necessarily in all cases bad or incorrect ( but that discussion , and the whys , hows and wheres of possible correctness is a WHOLE nuther discussion and immaterial to this discussion) so my first reflex is usually not to go after circular logic for being circular logic as such.. but in this case the shoe certainly fits.

For who could be free when every other man's humour might domineer over him? - John Locke (2nd Treatise, sect 57)