Descartes used the cogito to make two points the epistemological point that introspection affords us absolute certainty of our existence, and the metaphysical point that subjects are thinking things logically distinct from bodies. Most philosophers accept Descartes’s epistemological claim but reject his metaphysical claim. I argue that we cannot do this if the cogito works, then subjects are non-physical. Although I refrain from endorsing an argument for dualism based on this conditional, I discuss how such an argument would differ from the conceivability arguments pursued by Descartes in the Sixth Meditation and by contemporary philosophers. Unlike those arguments, this argument would not be refuted by the discovery of a posteriori identities between physical and phenomenological properties. In other words, it is possible to argue for substance dualism even if phenomenal properties are physical properties.