Physicians in ancient Greece believed four humours flowed within the human body-blood, phlegm, black bile, and choler-determining a person's health, mood, and character. Not until the seventeenth century would a more complex view of the anatomy begin to emerge. But by then humoural theory had already become deeply ingrained in Western language and thought-and endures to this day in surprising ways. Interweaving the histories of medicine, science, psychology, and philosophy, Passions and Tempers explores the uncanny persistence of these variable, invisible fluids. It will change how we view our physical, mental, and emotional selves.