A Certain Circularity
There is a grand circularity in the overall scheme wherein Descartes tries to find clear and convincing proofs of basic truths in order to provide solid foundations for his own Xtian world view and, by extension, for the truths of the mathematical and experimental sciences. This circularity is not vicious and Descartes himself was rather aware that it existed. Nevertheless, we should be aware of the circularity and how it affects the overall argument.
Descartes attempts to establish three basic truths by means of clear and distinct ideas and clear and convincing proofs. These are his own existence, the immateriality of the soul and the existence of God. The proof of the last basic truth supports other truths that, without the existence of God, cannot be rescued from radical doubt. These are the real (i.e. as more than merely the figments of a dream) existence of his body, the earth, the stars etc. For one of the characteristics of the Xtian and, by extension, the Cartesian God is that he is infinitely good. And, according to Descartes, an infinitely good God cannot allow us to believe that those things exist "in reality" if in fact they do not.
The circularity comes in when Descartes attributes the clarity and certainty of his three foundational proofs to God for the same reason. This is most clearly stated in the Discours de la méthode (p. 151 ff.): “…que les choses que nous concevons très clairement et très disinctement sont toutes vraies, n’est assuré qu’à cause que Dieu est ou existe….??? That is, the conclusion of the third foundational proof guarantees the validity of all three foundational proofs.
At a certain level this circularity is only apparent, for the existence (and goodness) of God are not part of any of the three proofs. When he invokes the goodness of God as the reason we find the foundational proofs to be clear and convincing, Descartes seems to be asking a genetic or investigative kind of question. In other words, the existence of God is not an assumption or step in any of the proofs. After the proofs have been established Descartes considers why those proofs are so clear and convincing and he looks to God for the reason.
As regards the proofs proper, Descartes’ basic argument is that anyone who denied any of their steps would be “extravagant??? or “déraisonnable.??? This argument has been used to justify the basic laws of logic from Aristotle and is one reason Descartes considers these proofs to be geometric. Of course, it is difficult to maintain that the authors of the Objections to the Meditations were unreasonable in quite the same way as someone who denies the law of non-contradiction. And the laws of both logic and geometry have had a rough time of it in the centuries following Descartes. Still circularity is not the ultimate problem with Descartes’ metaphysics. Rather, each one of his basic arguments suffers from radical internal fallacies.