He eventually discovers that “I exist” is impossible to doubt and is, therefore, absolutely certain. It is from this point that Descartes proceeds to demonstrate God's existence and that God cannot be a deceiver.

What does Descartes doubted?

In the first half of the 17th century, the French Rationalist René Descartes used methodic doubt to reach certain knowledge of self-existence in the act of thinking, expressed in the indubitable proposition cogito, ergo sum (“I think, therefore I am”).

What role does God play for Descartes and his doubts?

What role does God play in Descartes philosophy? ... The answer to all these doubts is that God is not a deceiver. Descartes' claim to know this rests on his idea of God. First, he uses the idea of God to prove that God exists in both the Trademark and ontological arguments.