This paper explains that Rene Descartes used doubt to prove his beliefs to be true as presented in his most famous work "Meditations on First Philosophy" in which he enters into his most radical phase of methodological doubt when he introduces his evil genius hypothesis. The author points out that Descartes uses the evil genius hypothesis to help illustrate his total abandonment of his old beliefs, but he does not start at scratch when he tears down his thoughts; he decides to adopt the opposite of the ones he held before he started his experiment. The paper relates that Descartes proves the existence of God by deciding that everything he clearly and distinctly perceives as true must be certain because having ascertained that he exists and that he is a thinking thing.
From the Paper:
"There is however, the difficulty raised with the case of deception by God, an example of this being geometry and arithmetic. These truths seem clear and distinct to Descartes, but there is still the possibility that he is deceived with respect to them. If God can deceive him of his clear and distinct perceptions, perhaps even the cogito can be cast back into doubt. Descartes seems to want to escape the problems involved in clear and distinct perceptions by relying on God's existence to make them true. However, Descartes also seems to want to prove God's existence by claiming it as a clear and distinct perception. Welcome to the Cartesian Circle."