Descartes (summary)

1. The demise of scholastic learning and the corresponding world-view with the emergence of a new, non-clerical intelligentsia in late-medieval Europe gradually discredited the authority of scholastic Aristotelianism, so Descartes, an outstanding representative of this new class of intellectuals, started out looking for different, self-sufficient grounds for certainty.
2. In this quest for certainty, Descartes’ methodical doubt, culminating in the Demon-argument, led to the need for finding an Archimedean point that withstands even this form of extreme, metaphysical skepticism.
3. Descartes found this in the certainty of cogito. Using the "clear and distinct" perception of the actual certainty of the cogito, Descartes then could set out to prove the existence of a benevolent, non-deceptive God, whose goodness guarantees even the habitual certainty in whatever is equally "clearly and distinctly" perceived, that is, in truths of reason.
4. Using an extended form of the Aristotelian principle of causality (whatever is or comes to be in the effect already has to be in some form of actuality in the cause), and a comparison of the objective reality of ideas, i.e., that reality-content of our ideas which they possess in relation to their objects in direct proportionality to the formal or actual reality these objects would have, if they existed (which of course Descartes cannot assume in his argument), he proves that there is a God, whose divine guarantee secures the formal truth of all a priori judgments (truths of reason), and consequently the material truth of all his clearly and distinctly perceived ideas, i.e., the innate, purely intellectual ideas of math, logic, and metaphysics.
5. Descartes’ argument can be spelled out in more detail as follows. It is certain “by the light of nature” that the formal reality of the cause has to be at least as great as the formal or objective reality of its effect. [Rf(C(E)) ≥ Rf,o(E)] But the idea of God has infinite objective reality. [Ro(I(G)) = ∞] Therefore, the cause of the idea of God has to have infinite formal reality. [Rf(C(I(G))) = ∞] But having infinite formal reality means being infinitely perfect, so the cause of the idea of God can only be an infinitely perfect being, that is, God himself. Therefore, God, who alone can be the cause of his idea in my mind, really has to exist. But then, the omnipotent deceiver of the Demon-argument cannot exist, since the only really existing omnipotent being is God, who, because of his infinite goodness cannot be a deceiver. Therefore, now I can rely on all those truths whose certainty was undermined by the Demon-doubt alone, namely, a priori truths of reason.
6. At this point, there are two main objections to Descartes’ process which we have to consider.
7. The "Cartesian Circle" is an objection against Descartes' using principles of reason in the process of recovering certainty, immediately after finding the "Archimedean point" for his system in the certainty of the cogito, i.e., the judgment that I think/exist. The objection can be stated in the following way. Any demonstration can be effective only if its premises are more certain than its conclusion. But as a result of the demon argument, Descartes cannot hold anything for certain, except the cogito. So after the demon argument no demonstration (of his) can be effective which uses premises other than the cogito. But Descartes’ demonstration of God’s existence uses other premises, such as the principle of causality (an effect cannot contain more [whether objective or formal] reality than the [formal] reality of its cause). So, since those premises cannot be more certain than the conclusion (as they cannot be certain at all), the demonstration cannot be effective. What constitutes the circularity here is that Descartes uses the proof of God’s existence to regain the certainty of principles of reason other than the cogito. But to do so he has to rely precisely on this kind of certainty in the proof itself, which therefore cannot be effective. In short, it seems that he wants to establish precisely the kind of certainty by his proof which he would already need for his proof to work.
8. Descartes’ reply involves distinguishing between two types of certainty we can have concerning truths of reason: (1) what I called "actual" certainty; the certainty we have when we actually consider the judgment in question, and clearly and distinctly perceive that it has to be true; (2) what I called "habitual" certainty; the certainty we have when we do not actually consider the judgment in question, but remember having considered it, and having once clearly and distinctly seen that it had to be true. Descartes claims that the demon argument can undermine only this latter, "habitual" certainty. Therefore, since in the proof for God’s existence we are actually considering the truth of reason stated as a premise, we do have -- the demon argument notwithstanding -- the actual certainty needed to render the proof effective. So the objection cannot show that the proof is ineffective.
9. Another objection, for which I think Descartes does not, indeed, cannot have an answer, is the "historical objection" I raised against Descartes’ method in general. Descartes’ reply to the circularity objection points toward a further, general weakness of the Cartesian method. For even in his reply Descartes simply assumes without further ado that the actual certainty of principles of reason is something that is provided by "the natural light", basically, the intuitive clarity of what he takes to be our innate ideas. But the very principle he uses there (the principle of causality in the form he states it), if it is clear at all, is obviously clear at best by the light of scholastic Aristotelian considerations, which Descartes is constantly using, whereas his whole enterprise was proclaimed to be based on the initial rejection (or at least suspension) of all such considerations. In fact, Descartes’ enterprise taken to the extreme, could be carried out consistently only by a Mowgli (the jungle boy raised by wolves), a purely biological human being, completely uninfluenced by previous social experiences and the consequent "prejudices" of the rest of mankind with a history. But it's quite certain that Mowgli would have better things to do than set out on such a Cartesian project.
10. In any case, assuming that Descartes did manage to prove the existence of a non-deceptive God, the divine guarantee of the material truth of all clearly and distinctly perceived ideas secures the material truth also of those sensible ideas whose objects can be perceived by means of the clearly and distinctly perceived intellectual ideas. This means that the sensible ideas of the mathematically and metaphysically clearly and distinctly perceivable features of material objects are also materially true, so at least this part of our sense perception is not deceptive either. There are material objects, which really have those mathematical features that they appear to have in sense perception. Regarding the other, less clearly perceived qualities of objects, however, we cannot have the same sort of certainty. But we still can have an understanding what these perceived qualities are in the things themselves, provided we are able to reduce them to the distinctly perceived primary, mathematical qualities, as their eminent causes. It is basically this distinction that underlies the general Cartesian, and even post-Cartesian "scientific" picture of reality, according to which, although we live in a world of sensible phenomena, this phenomenal reality may be utterly delusive, yet, by using science we may, as it were, "peek behind" this smoke-screen of phenomena, revealing the true nature of what there really is. That this picture is flawed in many ways was soon pointed out by several post-Cartesian philosophers, especially the British empiricists (Locke, Berkeley, Hume), which, however, does not prevent it from being very influential especially among scientists even nowadays.