Descartes

[Bad Joke Warning]:

Why can't you teach philosophy to horses?

Because you can't put Des Cartes before Des horse!

The fast food server asked Descartes, "Do you want fries with that?" Descartes said, "I think not," and vanished in a puff of logic.

Okay, now that those are over with, on to the actual philosopher. We have most of his Meditations in our text. It was quite an interesting read, with some of the same flavor as the readings from Augustus. It also contains an "ontological argument," i.e. an attempt to prove the existence of god starting from known principles.

I've seen various versions of such arguments dissected, but none of the dissections I remembered really seemed to get at the heart of Descartes' argument. There's an almost Platonic assumption in his thinking. Essentially, if you can think of something, then there must be something in the real world that connects to that thought. He mentions that things like unicorns and hippogriphs are mentally "constructed" from other "real" creatures. A painting of something even more fantastic would still use "real" colors. And since Descartes has a conception of an infinite being, there must really be something infinite out in the "real" world. He can also conceive of a necessarily existent being, so one must necessarily exist.

Why this necessarily existent being must be infinite, or have any other property that Descartes wants to attach to his god, isn't made clear in the sections of the Meditations in our book. This site does discuss why he would think that an omnipotent being would be a necessarily existent being:

To illustrate this point Descartes appeals to divine omnipotence. He thinks that we cannot conceive an omnipotent being except as existing. Descartes' illustration presupposes the traditional, medieval understanding of "necessary existence." When speaking of this divine attribute, he sometimes uses the term "existence" simpliciter as shorthand. But in his more careful pronouncements he always insists on the phrase "necessary and eternal existence," which resonates with tradition. Medieval, scholastic philosophers often spoke of God as the sole "necessary being," by which they meant a being who depends only on himself for his existence (a se esse). This is the notion of "aseity" or self-existence. Since such a being does not depend on anything else for its existence, he has neither a beginning nor an end, but is eternal. Returning to the discussion in the First Replies, one can see how omnipotence is linked conceptually to necessary existence in this traditional sense. An omnipotent or all-powerful being does not depend ontologically on anything (for if it did then it would not be omnipotent). It exists by its own power

Okay, so since he can conceive of an omnipotent being, one must exist and have "self-existence." It does not follow that there is no other self-existent being, nor that self-existence requires omnipotence. Even if it did, I've never found the whole 'Platonic Forms' thing remotely convincing, so "conceiving" of something has no connection to its "existence" for me. Also, I'm not convinced that I can conceive of an omnipotent being. I can imagine something very powerful, but not one with no limits whatsoever. And while I have a mental construct of a concept of "infinity," I would not say that I can genuinely imagine infinity.

But if you want more informed opinions on Descartes and ontological arguments, I found two good articles at the Stanford Encyclopedia of Philosophy:
Descartes' Ontological Argument
Ontological Arguments

My impressions are that the author of the first one finds Descartes' argument compelling, but there is still a decent discussion of the criticisms. The second one seems to be written by someone not convinced by any of the arguments, who points out that they all rest on premises that a non-theist would automatically question, and hence not be convincing to a non-theist.