Fides Quaerens Intellectum: Descartes's Ontological Arguments: "Peter Kreeft mapped out the steps of Descartes argument this way, which (I will say for now) seems right:

1. We have ideas of many things.
2. These ideas must originate from ourselves or from things outside us.
3. One idea we have is that of God—a being who is infinite and perfect in all respects.
4. The idea could not have been caused by ourselves because we know ourselves to be limited and imperfect, and no effect can be greater than its cause.
5. Therefore, the idea must have been caused by something outside us which nothing less than the qualities contained in the idea of God.
6. But only God himself has those qualities.
7. Therefore God himself must be the cause of the idea we have of him.
8. Therefore God exists."