How did the ancient Greek theory of four bodily humors explain health and illness?

The ancient Greek theory of the four bodily humors—blood, phlegm, yellow bile, and black bile—provided a foundational framework for understanding health and illness by linking human physiology to the four elements of the universe: air, water, fire, and earth.123

Health and Illness

According to this doctrine, health was defined as a state of equilibrium, where these four humors were perfectly mingled in terms of their compounding, power, and bulk. Illness, or pain, was believed to occur when this balance was disrupted—specifically when one humor became excessive, deficient, or isolated within the body.4

Humoral Associations

Each humor was associated with specific physical qualities and temperaments:56
* Blood: Associated with air and a sanguine (optimistic/excitable) temperament.5
* Phlegm: Associated with water and a phlegmatic (stolid/apathetic) temperament.25
* Yellow Bile: Associated with fire and a choleric (irritable/angry) temperament.5
* Black Bile: Associated with earth and a melancholic (sad) temperament.5

Physicians in the Hippocratic and later Galenic traditions used this theory to diagnose ailments, often attributing physical and mental conditions to specific humoral imbalances. This belief system persisted for centuries, frequently leading to medical practices like bloodletting intended to restore the body's natural harmony by removing "excess" humors.461