When someone yawns or weeps, it often causes others to do the same. In the same way, shuddering, hunger, and sexual desire are all transmissible from one person to another. This book addresses how emotions can be contagious, spreading rapidly among people and resulting in physical effects.

Medieval Latin had a specific name for this contagion: compassio ('compassion'). Etymologically, compassion means experiencing together the same 'passion', implying an involuntary reaction of the soul or the body imitating the reactions of others. This compassion-like mechanism intrigued medieval scholars. Ideas about it were examined in the fourteenth century by a series of savants commentating on Aristotle's Problems, and their notions held a particular place in scientific discourse. To this day, emotional contagion remains a topic of scientific enquiry and raises a number of philosophical, scientific, and anthropological questions. Emotional contagion is mysterious, and so it was in the Middle Ages. How could a single passion be both active and interactive, and what role could be assigned to the outside world in these emotional movements?

This book sheds light on how medieval scholars conceived of the soul-body relationship and human interrelations and illuminates the twofold enigma: that of the trajectory of the term compassio, and that of explaining the phenomenon it denoted.