In constructing the concept of truth I believe to be implicit in Sartre's philosophy, I shall inevitably make glancing contact with differences between his and Descartes' understanding of the cogito—their common foundational truth. This family quarrel will, however, remain peripheral to my discussion of what I take to be the core claim of any cogito-argument: that my existence is certain insofar as it results from my thinking. Beyond that, nothing is certain about what Descartes meant by the certainty of the cogito, or about what could reasonably be meant by it. His express inconsistencies in characterizing the (now inferential, now intuitive, even performative) force of his cogito-argument are well known, as are the various incompatible, textually more or less defensible, construals of generations of his interpreters. Unsurprisingly, then, the problem of truth for those of his followers, including Sartre, who have not been content to accept cogito, ergo sum as an unanalyzable datum, has tended to centre on proposing more intellectually transparent readings of their ancestor's seemingly inscrutable ergo.