I'm going to suggest an alternate answer to Jo Wehler's answer.
- Descartes tries to test the limits of doubt in Meditation 1. (a) First, the material world is suspect because we know our perceptions have been wrong before. (b) But our concepts are more certain (simplified version of the wax argument). (c) The apex of doubt is that nothing can be known -- not even our thoughts, because even the thoughts I have are being scrambled actively by the evil demon.
- To break out of position 1(c), Descartes asserts there's a good God. And then proceeds from 1(b) to conclude that de minimis he exists as a thinking thing (not to be confused with a fully robust rational animal human).
- (a) From 2, Descartes looks at his ideas and notes the idea of God (as quoted by Jo Wehler in his answer). (b) Descartes posits that ideas can come in three forms: (i) innate - built into him, (ii) adventitous - coming from the outside, and (iii) composite - put together from other ideas from (i) and/or (ii). (c) Thus, whatever the idea of God is it must be (i), (ii), or (iii). (d) Moreover, Descartes posits that an idea can only be generated from a sufficient source. (e) given the attributes of God in 3(a), the idea of God must either have been planted by a sufficient source or arrived adventitiously from such a source. (f) spelling this out, the only possible source regardless of mode for an idea as infinite and perfect as God is God.
- (same as Jo's 3): God's existence guarantees the truth of my clear and distinct ideas.
- This then applies to the mind first and the body later.
I take the argument to be more cosmological than ontological. I define an ontological argument as an argument based solely on considering the idea of God. I take Anselm to be the template: the idea of God is perfect in every respect. An idea that includes existing is better than one that does not. Ergo, the idea of God includes that God exists. Ergo, God exists. As I read Anselm, it's not important that someone think this for it to happen or that someone possess this idea. (note that we don't need to go all the way to Kant to find a rejection -- Aquinas thinks this doesn't work).
In contrast, I take a cosmological argument to proceed on the basis that some feature of the created order implies there's a God. Thus, the argument from cause raised by Aristotle and Aquinas is that (a) causality happens. (b) cause = A -> B. (c) There must be an A that has no recursive A. (d) that A is definitionally God. This argument hinges on the factual occurrence of (a).
I take Descartes' argument to actually be more in line with the cosmological form. Yes, it is about an idea, but it is not that the mere content of the idea produces an existing entity. Instead, it is about the origin of that idea and has precisely the A -> B linkage between an idea of God and some external cause sufficient to that idea.
When I used to teach this text, I raised two or three objections. One was similar to the one Jo raises. Another was that we now have machines that seem to be able to produce things greater than themselves (i.e.. several super computers joined together).