This essay, using works by Maurice Merleau-Ponty, Hazel Barnes, and Elizabeth Fallaize, documents the correspondence between the philosophical content of Beauvoir's She Came to Stay and Sartre's Being and Nothingness (both originally published in 1943). After reviewing the existential/phenomenological philosophical method, this paper examines the two philosophers' letters and diaries to show that Beauvoir wrote her book before Sartre wrote his and that the distinctive ideas and arguments the two works share originated with Beauvoir.