This article contributes to current discussions of the origins of modern sexuality by exploring an episode from Book II of Jean-Jacques Rousseau’s Confessions (1782), in the context of eighteenth-century childhood hygiene theory. In this episode, the sixteen-year-old Rousseau is the object of an aggressive sexual advance on the part of another young man, and witnesses this young man’s orgasm. By stressing his unusual immunity to understanding the sexual nature of this encounter, Rousseau places himself outside the cultural argument that moral degradation was bringing on premature puberty (ejaculation, in males) and thus degradation of the human species in eighteenth-century Europe.