Cartesians and Lichtenbergians have diverging views of the deliverances of introspection. According to the Cartesians, a rational subject, competent with the relevant concepts, can come to know that he or she thinks – hence, that he or she exists – on the sole basis of his or her introspective awareness of his or her conscious thinking. According to the Lichtenbergians, this is not possible. This paper offers a defence of the Lichtenbergian position using Peacocke and Campbell's recent exchange on Descartes'scogitoas a framework for discussion. A thought-experiment will be presented involving two communities with radically different conceptions of the metaphysics of the self. The purpose of the thought-experiment is to suggest that a substantive metaphysical thesis, whose truth cannot be a priori known, is presupposed by any justified transition from one's introspective awareness of a certain mental activity to the self-ascription of that activity.