Descartes' Meditations are an attempt to discover what we can prove that we know accurately. He begins by removing all that can be thrown into doubt, and this leads to him believing that nothing can be proved. However, in the second Meditation, he goes on to realise that because he is able to doubt the existence of things, indeed because he is thinking at all, he must have some form of existence. The famous phrase ``cogito ergo sum'' becomes his bedrock truth. Nothing can stop that being true. From this point, he goes on to try and build up other knowledge about the exterior world. At this point he runs into a problem. Having already determined that the senses cannot be assumed to be accurate, he has no means to gain knowledge of the world. He solves this problem through the use of God. Since God is benevolent, Descartes would claim, he would not allow it to be the case that we would be deceived in everything. Thus, there must be some things that are guaranteed accurate. These Descartes terms ``clear and distinct ideas''. Ideas that seem obviously true to us are in fact true. This is partly because they would only be clear and distinct ideas if they were true, and partly because God ensures that what we perceive clearly and distinctly is true. These ideas are almost certainly only those that are innate, since the senses are too inaccurate and untrustworthy. Thus, ideas about mathematics in particular would be clear and distinct, since they are possibly the most abstract anyway.
The commonly recognised flaw in Descartes argument is concerning his proof for the existence of God. The problem seems to be that Descartes believes it self-evident that God exists, so he gives only passing mention to his proof for God's existence. His argument states that since we have a clear and distinct idea of God, he must exist, clear and distinct ideas being true. The problem is that this has formed the Cartesian circle, as noted by Arnauld. God ensures the validity of clear and distinct ideas, yet God is assumed to exist because he is a clear and distinct idea. Clearly, better proof of either one or the other argument is needed to ensure its stability.
First, let us try to shore up the existence of God. One of the arguments proposed to do this is the ontological argument. This states that since God is a perfect being, it is inherent within perfection that God exists. Thus, God exists. The problem is that this uses very similar reasoning to Descartes, and thus is susceptible to the same attack. The argument uses the existence of God to prove the existence of God. Should one for a moment doubt either the perfection of God or the existence of a perfect being, the argument no longer has any foundation. If God doesn't exist, then he is not a perfect being, therefore there is no necessity for him to exist. If there is no perfect being in existence, then there is no requirement for God to exist. These counter-arguments seem a little overstated, but that is because they are trying to oppose invalid arguments.
Another argument for the existence of God is the `Maker's Mark' argument. This states that, since we all have a concept of God, and God being infinite, we as finite beings cannot be the origin of this concept. This argument is at least logically coherent, but the assumptions it makes are very much open to attack. To begin with, not everyone has a conception of God, or at least not the same perception of God. Were you to take the viewpoint of a Hindi, you would find that their concepts of deities are incompatible with the traditional Christian one. Also, should someone be unable to conceive of God, does that mean that God genuinely does not exist for him or her? It seems unlikely that this could be the case, but then how many people does it take to have this conception of God for it to be universally true? Indeed, the very fact that not everyone has the concept of God indicates that he is not a perfect or infinite being, and thus the concepts that the argument assumes to be held by all are false. Returning to the question of humans as finite beings conceiving of the infinite being we call God, it seems problematic to say that whilst we cannot conceive the idea of 'infinite', we can in fact comprehend it. I believe that our concept of infinite is in fact not infinite at all. It merely seems infinite to us, since we can in fact have no conception of what is genuinely infinite. Thus, if our conception of infinite is not in fact infinite, our conception of God as infinite is false, and there is no reason why we cannot have created the conception of him.
Having failed to prove the existence of God, and thus ensure the validity of all clear and distinct ideas, we can now, thanks to the circular nature of Descartes argument, attempt to prove the validity of clear and distinct ideas. This will therefore also prove the existence of God, thus neatly breaking the circle, and still ensuring that any arguments based upon either of the components of the circle can still be maintained. Perhaps the simplest argument in this direction is that clear and distinct ideas are clear and distinct because they are true, and therefore, anything that is a clear and distinct idea is, by necessity, true. We have somehow developed the ability to perceive that which is true clearly and distinctly, or perhaps the qualities of anything that is true are such that we perceive it clearly and distinctly. Such is the case with his proof of his own existence, and since that seems to be obviously true, it would have to be classed as a clear and distinct idea. Through this, Descartes suggests that all clear and distinct ideas must be true, since otherwise they could throw existence into doubt, and that cannot be done. This is an excellent example of how Descartes uses his new-found firm foundations to build other knowledge upon.
To answer the question fully then, the argument is most definitely `plagued' by circularity, but that does not necessarily destroy it. However, the circularity does seem to be a vicious circle, thus breaking down the argument. Having failed to find a proof for the existence of God, we no longer had any reason to assume that clear and distinct ideas are true. However, since we did in fact manage to sustain the idea that clear and distinct ideas could well be true, it is possible to use the Cartesian circle to our advantage, and claim that God exists, should one assume that he is a clear and distinct idea in everyone. This is perhaps the next question that must be raised. Does everyone have a clear and distinct idea of God, and if so, are they all the same. It would seem that the answer to both of these questions is no, which seems to once more deny us proof of God's existence. Nonetheless, clear and distinct ideas manage to survive, so the circularity was not too vicious.
J. Cottingham, The Cambridge Companion to Descartes (Cambridge: Cambridge University Press, 1992)
J. Cottingham, Western Philosophy: An Anthology (Oxford: Blackwell Publishers, 1997)
H.G. Frankfurt, Demons, Dreamers and Madmen: the defense of reason in Descartes' Meditations (1970)
C. Furlong, Descartes (http://www.geocities.com/Athens/Forum/5507/descartes.html)
B. Russell, History of Western Philosophy (London: Routledge, 1995)
Britanica Online (http://www.eb.com/)
Stanford Encyclopaedia of Philosophy (http://plato.stanford.edu/)
The Internet Encyclopaedia of Philosophy (http://www.utm.edu/research/iep/)
The Catholic Encyclopaedia (http://www.knight.org/advent/cathen/)
The Cartesian Circle (http://www-rci.rutgers.edu/ owl/descartes1.html)