Descartes proved the existence of God several times in his writings, but unfortunately the proof for which he is best known is his worst proof. It is of course not surprising since it was his first proof; according to his description, the Discourse on Method was a prototype for Meditations on First Philosophy. This proof, somewhat improved upon in the Meditations, was basically an ontological proof, and as has been observed (by Saint Thomas Aquinas, I have heard), the only one to whom an ontological proof for God is really viable is God Himself. What seems widely missed, however, is that Descartes also employed a variant of the proof from contingency.
This is actually a very natural thing for Descartes to have done, given what he had established as known at this point in the Meditations. He knew something existed which is capable of thoughts, and that these thoughts exist as successions in time. They change, and thus clearly are contingent. A contingent thing is all that’s really necessary for proving God, so why bother dragging in an argument about actual conceptions of infinite perfection which in any event it would be easy to argue no one actually has? My contention is that Descartes included this in the Meditations simply because he got attached to it having labored so hard on it in the Discourse. It thus forms the bulk of the words in his proof for God in meditation 3, but I maintain that one can simply excise the ontological proof found in meditation 3 and you are left with an intact proof from contingency. Accordingly, I will reproduce the relevant parts of meditation 3, without adding anything, but omitting all of the extraneous bits. I am here assuming that the reader possesses a copy of the Meditations on First Philosophy, and has already read it, so for brevity I am not going to reproduce what I have excised. (Cogito ergo sum, it will be recalled, came in meditation 2, so that will also not here appear in the quoted material.)
Now, it is manifest by the natural light that there must at least be as much reality in the efficient and total cause as in its effect; for whence can the effect draw its reality if not from its cause? And how could the cause communicate to it this reality unless it possessed it in itself? And hence it follows… what is cannot be produced by what is not… I am here desirous to inquire further, whether I… could exist supposing there were no God.
And I ask, from whom could I, in that case, derive my existence? Perhaps from myself, or from my parents, or from some other cause less perfect than God… But if I (were independent of every other existence, and) were myself the author of my being, I should doubt of nothing, I should desire nothing, and, in fine, no perfection would be awanting to me; for I should… thus be God… And though I were to suppose that I always was as I now am, I should not, on this ground, escape the force of these reasonings, since it would not follow, even on this supposition, that no author of my existence needed to be sought after. For the whole time of my life may be divided into an infinity of parts, each of which is in no way dependent on any other; and accordingly, because I was in existence a short time ago, it does not follow that I must now exist unless in this moment some cause create me anew as it were, that is, conserve me. In truth, it is perfectly clear and evident to all who will attentively consider the nature of duration, that the conservation of a substance, in each moment of its duration, requires the same power and act that would be necessary to create it, supposing it were not yet in existence; so that it is manifestly a dictate of the natural light that conservation and creation differ merely in respect of our mode of thinking (and not in reality).
All that is here required, therefore, is that I interrogate myself to discover whether I possess any power by means of which I can bring it about that I, who now am, shall exist a moment afterward: for, since I am merely a thinking thing (or since, at least, the precise question, in the meantime, is only of that part of myself), if such a power resided in me, I should, without doubt, be conscious of it; but I am conscious of no such power, and thereby I manifestly know that I am dependent upon some being different from myself.
But perhaps the being upon whom I am dependent is not God, and I have been produced either by my parents, or by some causes less perfect than Deity… Then it may again be inquired whether this cause owes its origin and existence to itself, or to some other cause. For if it be self-existent, it follows, from what I have before laid down, that this cause is God… But if it owe its existence to another cause than itself, we demand again, for a similar reason, whether this second cause exists of itself or through some other, until, from stage to stage, we at length arrive at an ultimate cause, which will be God.
And it is quite manifest that in this matter there can be no infinite regress of causes, seeing that the question raised respects not so much the cause which once produced me, as that by which I am at this present moment conserved.
I think that it is possible to get side-tracked by Descartes observations about the power to create and the power to maintain being the same power. I think he is largely correct, but it is a topic not often remarked on. The alternative is more or less silly if you don’t employ physical metaphors like dominoes to mask over what’s really being said. In an eternal Godless universe, each moment contains the existence of all subsequent moments to it, but does not contain its own existence. It has an infinitude of causes, but not its own. So when it hands on this infinitude of causes to the next moment, it somehow subtracts the cause of that moment before handing it on. It’s an infinite chain of parents who live by eating their children. The more common objection of atheists for a Godless world without a beginning always smuggles in some common cause between moments, or just in changes—such as space and gravity in the case of the dominoes metaphor—but if you subtract the smuggled in common independent cause, it amounts, in essence, to summing zero an infinite number of times to try to get a positive number. Or as often as not to pure nonsense where words are used as a smokescreen: one can always say, “but what if it doesn’t?” in response to anything, without having any conception of a coherent alternative.
In any event, while Descartes was certainly fond of his ontological proof, I think a careful reading of the above material will show that the real spine of his proof is the proof from contingency. The parts I excised in the ellipses serves, I think, to anticipate and counter the objection which I have run into, “why does the non-contingent thing need to be intelligent? Why can’t intelligence be an emergent property?” Rather than use the substance of free will and rational thought as an answer, as Saint Thomas did in the Summa Theologica, Descartes tried to use the concept of God—which he maintained could not come as an inference from sense perception in the manner Hume would later insist all knowledge did—to prove that there was something which emergent properties could not account for. I think that this is unnecessary as all people have a direct experience of cognition and a very indirect experience of material causality, and so the objection can be dealt with merely by pointing out that the objector doesn’t believe the intelligence they have can be the result of emergent properties any more than anyone else does, but Descartes may simply have been more generous (or more ambitious) in dealing with the sort of people who would argue that emergent properties are the origin of everything immaterial.
Of course, I have no better window into Descartes’ soul than does any other human being, and the epitaph on his grave which he requested (“he who hid well, lived well”) certainly is a point to the side of those who thought Descartes intended to start Modern Philosophy. My point is not really to weigh in on that debate. Rather, it’s that Descartes certainly produced a good (in the sense of coherent and valid) proof of the existence of God, even if he let it get snowed under by his attachment to the ontological proof and anticipation of a myriad of not very sophisticated objections. Thus there is no middle ground between him having innocently wrecked the world, and him having been a villain on par with Iago.