The problem of the Cartesian circle, as it is called, has sparked ongoing debate, which intersects several important themes of the Meditations. Discussions of the circle must address questions about the force and scope of the famous method of doubt introduced in Meditation I, and they must examine the intricate arguments for the existence of God and the avoidance of error in Meditations III to V. These discussions raise questions about the possibility of overturning skepticism, once a skeptical doubt has (...) been introduced. More generally, the problem of the circle resonates with recent questions about the foundations of knowledge: Must we be able to validate our methods of reasoning or of knowing before using them? If we must, wouldn't we be forever stuck at the beginning, unable to use our methods of reasoning or of knowing in their own validation? The problem of the Cartesian circle raises general questions about the validation of reason and the possibility of knowledge. This chapter examines the Cartesian circle in the context of Descartes' central project in the Meditations, to secure the foundations of metaphysics. In carrying out this project, Descartes felt the need, or adopted the strategy, of examining the possibility of human knowledge more generally. Such an examination can be interpreted in various ways. Depending on the interpretation given, the roles assigned to the method of doubt and the proofs about God may differ, thereby altering how we see the problem of the circle. We therefore need first to consider Descartes' metaphysical project along with the methods and strategies he adopted in carrying it out. Subsequently, I explain and evaluate several main approaches to the problem of the circle. The chapter concludes with some reflections on the relation of Descartes' metaphysics of knowledge to other prominent positions in the history of philosophy. (shrink)