This dissertation analyzes one eighteenth-century French philosophe’s proposal to educate girls to be very sincere as a way of rectifying social injustices built into the economic and legal systems of Ancien régime France. Louise d’Épinay (1726-1783) was the author of a novel, educational works, a prodigious correspondence with leading Enlightenment thinkers, and contributions to the clandestine literary journal the Correspondance littéraire, read by European heads of state. I argue that many of d’Épinay’s proposals for changing French society have been overlooked until now because one of her major works, L’Histoire de Madame de Montbrillant, a nearly 2,000-page epistolary novel likely written over several years beginning in 1756, has been misread for over two centuries. Critics have long considered Montbrillant a thinly-disguised and scandalous memoir. Through close reading and study of the manuscripts of this work, I show that it is better understood as a roman pédagogique (or educational novel), according to the characteristics of the genre set forth by Robert Granderoute. Read as such, d’Épinay’s novel is just as much about pedagogy as those of her works traditionally classified as educational, Les Lettres à mon fils and Les Conversations d’Émilie. Read as a roman pédagogique, we can see that Montbrillant contains a curriculum for girls’ education that was radical for its time. Many critics have presumed that d’Épinay got the majority of her educational ideas from her well-known contemporary Jean-Jacques Rousseau and other male contemporaries. I argue that d’Épinay’s educational curriculum, based on intensive reading, writing, conversation, and introspection, reinvents for the Enlightenment a philosophical tradition we can trace back to the great seventeenth-century French letter-writer Madame de Sévigné. Thus, Montbrillant is part of a network of works by women who looked to other women for reflections on education. D’Épinay thought girls were as capable as boys of using reason, reading and writing, learning from direct experience, and grappling with abstract concepts. Moreover, she thought girls must learn to do all of these things, both for their own happiness and that of their future husbands. Montbrillant champions a new type of companionate marriage based on intellectual and moral compatibility between two spouses rather than the size of the woman’s dowry. The novel also offers practical guidance for its female readers by including examples of legal documents such as a separation of assets, as well as scenes that illustrate how women can maximize their economic and legal agency within the framework of a system that is structurally biased against them. D’Épinay rejects conventional educational methods – sending girls off to a convent and boys to a collège with a précepteur –, she shows mothers that they must secure the right to teach their children at home in order to prepare them for this new type of marriage, and she demonstrates ways they can do so effectively. Above all, d’Épinay advocates teaching girls to be sincere using forms of Catholic spirituality, stripped of their religious content, including a daily examination of conscience and reliance on a secular “spiritual” director for guidance. She believes that not only will their sincerity be worth more than a dowry to those worthy men capable of recognizing it, it is also the quality that will transform marriage from an unequal, often abusive hierarchy into a partnership of equals and that will allow women to acknowledge their common oppression. My reading shows that d’Épinay’s educational curriculum was meant not only to change the foundations of the family but also to transform a corrupt state nonviolently, an alternative to revolution. Montbrillant must be understood as part of a larger project of d’Épinay and her collaborator Friedrich Melchior Grimm’s – to broadcast a new form of education based on the love of virtue to élites who could promote it in their states. My analysis stresses the seminal importance of d’Épinay’s educational theories, no longer relegated to the shadows of Rousseau but instead revealed as an innovative educational curriculum meant to help women work around the constraints of the existing marriage system and, ultimately, to change that system. D’Épinay’s proposals make hers one of the most important feminist voices of her era, and my work provides an alternative literary history and a new understanding of Enlightenment culture and women’s contributions to it.