This is the first modern edition of Book X of the Historia Animalium. It argues that the first five chapters are a summary, from the hand of Aristotle, of a medical treatise by a physician practicing in the fourth-century BCE. This gives short shrift to Hippocratic staples such as trapped menses and the wandering womb, and describes a woman's climax during sex in terms that can be easily mapped onto modern accounts. In summarizing the treatise and examining its claims in the last two chapters, Aristotle follows the method described in the Topics for a philosopher embarking on a new field of study. Here we see Aristotle's ruminations over the conundrum of a woman's contribution to conception at an early stage in the development of his theory of reproduction. Far from being an insignificant pseudepigraphon, this is a central text for understanding the development of ancient gynaecology and Aristotelian methodology.