The Four Humours

Humoral theory holds that the body is made of four humours — blood, phlegm, yellow bile, and black bile — whose proper mixture (krasis) is health and whose excess, shortage, or separation is disease. Its clearest early statement is the Hippocratic treatise 'On the Nature of Man' (late 5th c. BCE), traditionally credited to Hippocrates' son-in-law Polybus, which linked the humours to the elements and the seasons. Galen (2nd c. CE) later wove the scheme into a full doctrine of temperaments, and this framework dominated medicine across the Greek, Islamic, and European worlds into the 19th century.