Abstract: Heidegger's Sein und Zeit (SZ) is commonly viewed as one of the 20th century's great anti-Cartesian works, usually because of its attack on the epistemology-driven dualism and mentalism of modern philosophy of mind or its apparent effort to ‘de-center the subject’ in order to privilege being or sociality over the individual. Most who stress one or other of these anti-Cartesian aspects of SZ, however, pay little attention to Heidegger's own direct engagement with Descartes, apart from the compressed discussion in SZ §§19–21. I here show through a careful reading of Heidegger's lectures on Descartes from the years immediately preceding SZ that, while he has sharp criticisms of Descartes and certain ‘Cartesian’ aspects of modern philosophy along the lines commonly recognized, he also aims to disclose what he calls the ‘positive possibilities’ in Descartes and the philosophy he inspired. I detail a number of these and then show that they force us to see Heidegger's own early project as largely unconcerned with dualism and mentalism per se, and much more with questions of the philosophical methodology that gives rise to them. Moreover, I show that a careful reading of Heidegger's treatment of the cogito makes clear that he is no serious way attempting to ‘de-center the subject’ and that the fundamental question of the ‘analytic of Dasein’ is one that takes Descartes as an immediate jumping off point: how can I articulate what I understand myself to be as the general kind of entity I am, and on what besides me does my being depend?