In the structure of a philosophical system, the form is at least as important as the content. Part of what I wish to demonstrate, but only by implication, is that Descartes' hold on later philosophy has a great deal to do with its form, and perhaps less with its content. One can quibble, for instance, about whether Descartes has given us an adequate proof of the existence of God, or even of the existence of the external world. One can doubt that his reasoning is sound about the mind being a distinct and unique substance in its own right as matter, or doubt that he is right about their essential properties being thought and extension, respectively. But it would not follow that therefore his way of doing philosophy is inadequate. Descartes has provided a form, a way, a mode of doing philosophy that has not yet disappeared even if no one takes seriously, for example, the idea that the pineal gland in the brain is the meeting place of mind and matter. This chapter is as much about the form of that kind of philosophizing as it is about the specific issues in Descartes' philosophy.

Let me, at least once, draw an explicit parallel between the form of philosophy in Descartes and the form of philosophy employed by some twentieth century philosophers of science: Descartes was after not only truth, but also certainty; contemporary philosophers of science are after truth or probable truth.