This chapter presents Descartes' method of doubt as consisting of various challenges to a series of "models of knowledge" with the aim of finding one that resists every conceivable challenge. In very allusive fashion, Descartes deploys the Pyrrhonian trope that what is perceived is relative to the conditions under which it is perceived and therefore lacks the objectivity that characterizes knowledge of the truth. In a later paper on the madman and the dream objections, Janet Broughton discusses at greater length why Descartes invoked lunacy in Meditation One. David Scott argues, on the contrary, that the madness hypothesis is taken seriously by Descartes, and its skeptical force is subsequently incorporated into the dream argument. The cogito must be immune in this way if permanent skepticism at the outset is to be avoided. Still, it is on certainty that God exists that certainty about the cogito, like all other certainty, depends.