In order to prove that a good God exists, Descartes must rely only on those beliefs he himself has defined as indubitable. His line of thinking is as follows: Descartes says that he finds the notion of a perfect being in his mind. The cause of this notion has to be at least as perfect and real as this notion is in Descartes’ mind. Further on, he states that he is not that perfect and nothing but a God, who is good and perfect, could possibly be the cause of this notion of a perfect being. Therefore, this God must exist.
The essential fault of this argument is the second premise. There is no reason to think that a thing’s cause is supposed to be as perfect or even better than its effect. It is not clear what perfection is even supposed to mean, just like it is not clear why causes have to be more perfect. However, even if it could be explained, this argument still presents not a belief about Descartes’ mind’s contents but rather a belief about the way things are beyond his mind’s borders.