THIS IS A BOOK about Descartes's method of doubt, about his rationale for using it and the way he thought it worked. Radical doubts surface in the Discourse on the Method and the Principles of Philosophy, and the method of doubt guides the fragment of the Search for Truth that has come down to us. But Descartes shows us the method of doubt most clearly in the Meditations on First Philosophy, and that is the book that will concern me in the chapters ahead.
In the first of his six meditations, Descartes offered the dream argument, which calls into doubt the existence of the things we see and touch, and the deceiving God argument, which in addition calls into doubt the truth of claims like “Two plus three equals five,” claims that we grasp “clearly and distinctly.” Descartes resolved to carry his meditations forward by affirming only what can withstand these radical skeptical arguments. He affirmed first that he himself exists and has various states of consciousness, or ideas. He argued that his own existence as someone with an idea of God requires that God exist, and then he appealed to God's benevolence for the validation of his clear and distinct ideas. From various of these clear and distinct ideas, he drew out the distinction between mind and body and the existence of a physical world describable in austerely mathematical terms.
When we reflect upon the trajectory of these meditations, we may find ourselves with some disturbing reactions: we may find it difficult to resist the radical skeptical arguments with which Descartes began, and yet impossible to accept his argument that God must exist if we have the idea of God. Then we seem to be left in need of something we cannot have: certainty that all of our most evident judgments are true. For the radical skeptical arguments seem to show that we cannot claim knowledge of anything beyond our own ideas unless we have something like a divine guarantee that our ideas correctly reflect the mind-independent world outside of our thought. The scope of our knowledge, then, appears to be tiny: each of us knows only his own existence and can attribute to himself only his states of consciousness. Those states we know through and through; that is our consolation prize. But everything outside this