This thesis is concerned with literary representations of the passions in a selection of eighteenth-century French epistolary fiction. In close readings of Charles-Louis de Secondat, Baron de Montesquieu’s 1721 Persian Letters, Francoise de Graffigny’s 1747 Letters of a Peruvian Woman and Jean-Jacques Rousseau’s 1761 Julie, or the New Heloise, I consider how the passions serve, in the context of narrative, to ground claims about human nature and sexual difference, and to speculate on the social and political apologetics at work within each text. The importance of sensibility in the culture and literature of the eighteenth century has to some extent eclipsed the role of the passions in literary representations of human nature and sexual difference. However central sensibility became to eighteenth-century imaginations, it did not eclipse but rather complemented developing moral and natural philosophical conceptions of the passions. As each of novels explored here attest, the passions remained central to literary and philosophical claims about human nature and sexual difference in the eighteenth century, providing a common vocabulary for making claims about the state of the social and political order. Entrenched, polysemous, and changing, discourses of the passions in Early Modern Europe served multiple and divergent ends. The goal of this thesis is to contextualize their representations in the narratives of three eighteenth-century novels as interventions in moral philosophy, shaped not only by epistemological philosophy but also the imperatives of a French literary tradition of gallantry.