The Fourth Meditation offers a proof that everything we clearly and distinctly perceive is true. It repeats that God’s perfection rules out deception (AT 53). It also has an argument that God would not have created us with unreliable intellectual faculties: that isn’t something that a perfect creator would do (AT 53-6).
The Fifth Meditation concerns our knowledge of the essences of both bodies (things like desks, chairs, cacti, cats, and so on) and God. The essence of body is extension; we know this through the intellect, rather than the senses. The essence of God is existence. This proves that God exists according to what is called the ontological argument.
Is God good?
I started by pointing out that I don’t think Descartes has proven that God would be good. Even if he has proven that God exists, why does it follow that God isn’t a deceiver? Maybe God thinks it’s funny to mess with us. Who knows?
Nathana suggested that Descartes might have an argument like the one he used in the Third Meditation in mind: the idea of a good, non-deceitful being must have come from outside of the human mind because human beings are bad and deceitful. Very clever.
I wonder if Justin’s point about the infinite being that has a horn in its head might not apply here as well: we do get the idea of honesty from other people. After all, some of us aren’t deceivers.
In any event, I don’t recall Descartes having said that. It’s a clever suggestion, though!
It should go without saying that God may well be perfectly good. All I’m saying is that Descartes hasn’t proven it.
Following Jeremy’s suggestion, we looked at AT 57-8, pp. 40-1. It’s an incredibly rich section.
There, you’ll find the claim that the human will is qualitatively identical with God’s: we decide between one alternative and another and form the correct judgments on the basis of reason just as God does. God knows more and has more power than we do, of course.
Note that Descartes does not think that being determined to believe the truth is incompatible with freedom.
Finally, Descartes gives an explanation of error that is very much like Augustine’s.
The essence of body is extension: measurable dimensions like length, breadth, and width. That means that the essential properties of bodies are geometrical: they’re shapes.
Furthermore, we know this by the intellect, not the senses. In discussing this, we finally got back to a question Nathana asked a long time ago: since we get the idea of triangles by seeing them, why does Descartes dismiss the senses? For Descartes’s answer, see AT 64-5, p. 45.