This essay aims to contribute to two rich and overlapping conversations in the history of medieval and early modern medicine in western Europe. First, it offers new perspectives on the ongoing discussion of the significance of physiology in premodern conceptions of the body, with a particular focus on sexual difference. Second, it enters into dialogue with a growing body of historiography concerned with the peculiarly flexible character of humoral epistemology: a highly successful theoretical framework for understanding the workings of human bodies that lasted for two millennia. These two conversations come together in our exploration of late medieval discussions of ‘breastbleeding’, or the phenomenon in which physicians understood menstrual blood as failing to properly transform into milk, therefore making the maternal body unable to lactate. The medical significance of this condition is presented in Hippocratic aphorism 5.40, one that has not garnered much scholarly attention. In this particular and theory-laden configuration, the breastbleeding of women is presented as a clinical sign of madness.

Transformation is key to humoral epistemology. The ability of bodies to transform substances explained how bodies worked, how they stayed healthy, and how the sexes expressed marked differences. In this essay, we explore humoral physicians’ imaginings of the inability of women's bodies to successfully transform blood into milk or to purge it entirely, which inability, they asserted, could trigger toxic or other harmful consequences.