His proof of the existence of god is about cause and effect.
The problem is that the cause - god is perfect - did not came out of nowhere.
According to Descartes, everyone has the idea of a perfect god inside him- or herself. That idea is not ours since human beings are imperfect. So god himself (being perfect) has caused that idea.
But I' ve been told about god since I was little so the idea came from my parents, teachers,... therefore I can' t belief his proof. If I never had heard about god before and if I had that idea then I would have believed it.
3. That the world as we know it exists.