In Sex, Love, and Letters. Writing Simone de Beauvoir, Judith Coffin investigates the reaction to Simone de Beauvoir’s Deuxième Sexe by Dominique Aury, the author of Histoire d’O under the pen name Pauline Réage. She highlights Aury’s scepticism about the feasibility of achieving a “union fraternelle entre les sexes”. I propose here to expand on Coffin’s analysis with a discussion of Une fille amoureuse, Réage’s preface to Retour à Roissy. In this preface, Réage introduces a mysterious person, whom she calls her “unknown speaker”, for whom she is simply the spokesperson. The work hypothesis I am suggesting is that, for Aury, it is the eternal presence of this mysterious person, who acts as an invisible invariant within the men and women, that impedes the project of a “union fraternelle entre les sexes”.