How should we understand both the nature, and the epistemic potential, of Descartes’s Cogito? Peter Slezak’s interpretation of the Cogito’s nature sees it strictly as a selfreferential kind of denial: Descartes cannot doubt that he is doubting. And what epistemic implications flow from this interpretation of the Cogito? We find that there is a consequent lack of knowledge being described by Descartes: on Cartesian grounds, indubitability is incompatible with knowing. Even as the Cogito halts doubt, therefore, it fails to be knowledge.