I think Descartes succeeds at proving the existence of God inside his own mind, but not elsewhere. His proof in the 5th Meditation goes: He can’t think of god except as existing, it follows that existence is inseparable from God, so therefore God really exists. His definition of God includes perfection, so he then says that something must exist in order to be perfect, and therefore God exists. My problem with this is that Descartes is trying to prove physical, corporeal existence from an idea that exists only in his head. In his first argument, he says he can’t think of God except as existing, meaning that his argument begins in his head. He then says that existence is a property of God (triangle analogy), therefore God must exist. This is where I think his argument falls apart. If you accept his logic for God’s existence, which I don’t, but if you do, he has done nothing more than prove that God exists inside his head. How does God escape the box of Descartes’ head? There is a difference between existence of an idea and existence as an idea. Descartes has only proven God’s existence as an idea. Descartes might object by saying that existence is built into the idea of perfection, and God is perfect, therefore he exists. But that still requires you to accept that God is perfect. Also, he never says how corporeal existence comes to be from perfection.
I’m not buying it. Are you?
The Practical Philosophy 