In his Meditations, Descartessets out to establish what he can know for sure. First, he recognizes that his senses occasionally deceive him, so sensory knowledge can’t be said to be certain. Then, he admits that sometimes his dreams are so vivid, he’s not sure whether he’s awake or asleep-how does he know for sure he’s not dreaming right now? But, he reasons, surely “whether I am awake or asleep, two and three add up to five, and a square has only four sides” (63). However, since he’s set out to make a clean sweep, to be rid of old opinions and habits of thought, to begin again from the very foundations, he then postulates an evil demon that may be deceiving him about everything. Does his thought experiment lead him to skepticism (the view that we can’t know anything about anything)? No. Instead, Descartes reasons as follows: “If he deceives me, then . . . I undoubtedly exist; let him deceive me as much as he may, he will never bring it about that, at the time of thinking . . . that I am something, I am in fact nothing. . . . ‘I am,’ ‘I exist,’ whenever I utter it or conceive it in my mind, is necessarily true” (67). Hence Descartes’s famous cogito ergo sum-I think, therefore I am.