How did Galen's ideas about the four humors influence medicine for centuries?

Galen’s influence on the history of medicine was profound, shaping medical theory and practice for approximately 1,400 years. Building upon earlier Hippocratic concepts, Galen posited that human health depended on maintaining an equilibrium among four bodily humours: blood, yellow bile, black bile, and phlegm.1

Unlike his predecessors, Galen argued that humoral imbalances could be localized to specific organs, not just the body as a whole. This modification allowed physicians to make more precise diagnoses and prescribe targeted remedies to restore balance. Because Galen’s physiological theories were highly systematic and difficult to challenge, they became the foundation of medical education in medieval universities and remained dominant in the Byzantine world and the Muslim Middle East for centuries.1

While his anatomical observations—often based on animal dissections—were eventually corrected during the Renaissance, his physiological framework persisted until the 17th century, when William Harvey correctly explained the circulation of the blood.1