This meditation completes Descartes project of reconstructing his beliefs. So far he has established only his beliefs about himself as a thinking thing and a belief in a perfect God. The aim of this last meditation is to extend his confidence to beliefs about the external world (including his own body).
• He begins (pp. 92f) with an initial argument based on the difference between his imagination and his understanding. Although he does not rest much on this argument, it is interesting at least for its form, as something that would now be called an “argument to the best explanation.”
• Following this, Descartes sets out a course of argument that lasts for the rest of the meditation. He outlines (near the top of p. 94) three parts to this discussion, though it is really the last of the three that forms the bulk of the meditation. He first (pp. 94f) considers the sources of his beliefs about the external world. You should pay attention to differences among these sources, especially in the extent to which they are dependent on his will. Next (pp. 95f) he reviews his doubts. Although this recounts the content of meditation 1, he now has some new ways of describing these doubts. Finally (near the top of p. 96, just before AT 78), he sets out to distinguish he what he can now believe from what he will continue to doubt.
• He begins (pp. 96f) by showing that at least some corporeal things exist, using an form of argument similar to the one with which the meditation began but also invoking the principle that God does not deceive. Since this allows him to believe in a world of extended substances obeying the laws of mathematics, it seems to give him grounds to accepting at least the general principles of his mathematical physics.
• To move beyond this and recapture ordinary particular beliefs about the world, he must distinguish what is to believed in what “nature teaches” him. In doing this, he first (pp. 97ff) distinguishes what he is really taught by nature from conclusions that depend on a “reckless” use of his will. However, he notes (pp. 99f) cases of error that are attributable more directly to what nature teaches us, and gives these an extended consideration in a series of four observations and conclusions drawn from them (pp. 101f). As Descartes’ final comments (p. 103) make clear, this discussion is designed both to further reconcile the existence of error with God’s perfection and to provide the means for assessing the degree of confidence that various beliefs warrant.