I believe in God. Unfortunately this thread of thinking is flawed on the point that nothing other than a divine beling could have place the idea of itself in man. Why? Man knows he is mortal. In some primal way, we all understand we are some day going to die, at least in the physical sense that we watch others die. Because man can think, and is aware of himself, he cannot easily imagine the void of being dead, of not being in some way conscious or alive. Believe in a higher deity is the next level up. Descartes is suggesting man couldn't have come up with the idea of God unless he/it actually exists. I think Descartes didn't give Man's initrinsic ability to reason and extrapolate the credit it deserves. Its a bad argument to use on the existance of God. Using it weakens that side of the debate.

The imaginations of men have come up with countless examples of absolutely fantastic, and absolutely fictional, stories. If a man can think up a large, firebreathing, scaled lizard (dragon) and a horselike creature with a single horn and undefined magical properties (unicorn), not to mention science-fiction and the other great works of the human imagination, this argument disproves itself, as man is obviously capable of imagining fictional beings greater than himself. Since this has occurred, your argument is refuted on the grounds that people can imagine fictional things, and sometimes even erroneously believe them to be true.