We identified four points that Descartes covers in the Third Meditation. We talked about three of them. Then we quickly mentioned two famous objections to Descartes’s project. The details are given on the handout.
The burden of the Third Meditation is to show that our idea of God is different from our other ideas. This idea is such that it must have an external cause that resembles the idea. That isn’t true of our other ideas: they could be generated by ourselves or things that they don’t resemble (like evil geniuses, say).
We went through some of the detail of Descartes’s argument for that conclusion. Very basically, it is that finite beings cannot generate ideas of infinite things on their own. We also went through how one might challenge that by showing how the idea of an infinite being could be generated by a finite one. It helps if the idea of an infinite being is couched largely in negative terms, such as “the idea of something greater than we can understand.”
But leave those objections aside. Suppose that Descartes has shown that the idea of God must be caused by a real, infinite thing. What does that have to do with establishing a foundation for the sciences? Descartes’s answer is that it would show that God is not a deceiver and would not allow us to be generally deceived. Therefore, we can trust our senses and other beliefs, free from the doubt engendered by the possibility that we are deceived by dreams or evil geniuses.
It’s that step that seems to involve some very substantial assumptions on Descartes’s part. Why think that an infinite being would care to ensure that we aren’t deceived? We might think that if we associate perfection with God and associate good qualities in God with good qualities in human beings. But why make those associations?
Perhaps Descartes was reading the image of God idea in the other direction. If we’re like God, then God is like us. Just as a good person isn’t deceptive, God must not be deceptive.