In the Second Meditation, Descartes arrives at his indubitable first principle: “I think, I exist” (AT VII 25), elsewhere stated as “I think, therefore I am.” What is the basis for Descartes’ knowledge of his own existence? Is it the result of an inference (as his use of “therefore” might suggest), or did he come to know it in some other way? If the former, how is his knowledge compatible with the possible existence of a deceiving God? If the latter, how is his knowledge to be understood?