Broughton first contrasts Descartes's doubts with those of the ancient skeptics, arguing that Cartesian doubt has a novel structure and a distinctive relation to the commonsense outlook of everyday life. She then argues that Descartes pursues absolute certainty by uncovering the conditions that make his radical doubt possible. She gives a unified account of how Descartes uses this strategy, first to find certainty about his own existence and then to argue that God exists. Drawing on this analysis, Broughton provides a new way to understand Descartes's insistence that he hasn't argued in a circle, and she measures his ambitions against those of contemporary philosophers who use transcendental arguments in their efforts to defeat skepticism. The book is a powerful contribution both to the history of philosophy and to current debates in epistemology.