Descartes's Two Arguments for the Existence of God
In building his edifice of knowledge Descartes uses two arguments for the existence of God. It is something of a paradox that he would attempt proofs of God's existence. After all, he claims that the “school men” and all prior philosophers have served up only doubtful opinions. Yet he undertakes two proofs, the same kind of proofs undertaken by Augustine in the fourth century to Thomas Aquinas in the thirteenth.
The Causal Argument for God's Existence
In the third meditation Descartes offers up an argument that he hopes will show that God is the source of all perfection. It could be labeled the causal argument, and a form of it appears in the Neo-Platonic reason of Augustine. Though Descartes recognizes him as an imperfect being, he is able to entertain the idea of God as a perfect being. Since he is incapable of generating such an idea on his own, there must be some greater cause of the idea. The argument relies on the Scholastic principle that there must be as much reality in the cause in as in the effect. So any ideas of perfection require perfect causes of them. Therefore God, the perfect cause, must exist.
One way of countering the argument would be to deny the premise and say that you do not have a perfect idea of God. Rather, you have only an approximation of that perfect idea. Since our idea of God would then be imperfect, it would not require a perfect cause and the conclusion wouldn't follow.
The Ontological Argument for God's Existence
Descartes's second argument for the existence of God occurs in the fifth meditation. Though the wording is slightly different, it recalls Anselm's ontological argument from late in the eleventh century. Here Descartes considers the idea of a most perfect being and what such an idea contains. If the being is truly perfect, then it would lack nothing. Therefore, it would not lack existence. So God's essence contains his existence. “We can no more think of God without existence than we can think of a mountain without a valley,” Descartes says.
The argument about God's existence following necessarily from his essence also has implications for the consideration of the evil deceiver raised in the first meditation. For if God is perfect, he is incapable of being a deceiver. He cannot lead anyone into error, either. Deceit and induced error would be inconsistent with the character of an imperfect being. Since he cannot lead you into error, you can attain knowledge of all those matters he doubted in meditation one, especially the truth about material objects.