Starting from the hypothesis that the Heideggerian and Sartrean philosophies give different analyses of personal identity and autobiography, this paper aims to define Sartre’s account regarding these issues, but also to explain why his phenomenological approach stands out from the analyses made by Heidegger. This study thus presents Heidegger’s conception of selfhood and its reception by Sartre in the context of the Second World War. It indicates the objections raised by the latter against Being and Time’s conception of the “ipseity of Dasein,” and how these remarks led him to invent a phenomenological method named “existential psychoanalysis,” whose function is to decipher everyone’s autobiography by examining the phenomenon of individuation from a double perspective: from a phenomenological standpoint that analyses the experiences of appropriation of facticity which are achieved by the consciousness through an intentional project, but also, from an “un-phenomenological” standpoint that consists in describing all the situations which shape the constitution of the project through which consciousness appropriates the contingency of its existence. As a result, Sartre suggests—unlike Heidegger—that a phenomenology of autobiography should take into account the relationships between the individual and society.