The doctrine of humors (from the Latin for liquid or fluid) refers to the ancient Greek theory of the four bodily fluids: blood, phlegm, choler (yellow bile) and melancholy (black bile) that determined health and temperament. Humoral theory formed the basis of western medicine and had tremendous influence up to the 19th century. The common practice of blood letting, for example, was intended to rid the body of excess humors. Even such innovators as Franciscus Sylvius (1614-1672), who examined the chemical basis of disease, still kept their systems within the general framework of a humoral explanation. William Harvey (1578-1657), who proved that blood circulated in the body, was attacked by many of his contemporaries for trying to demolish established ideas.

Humoral theory was an extension of the earlier writings of Empedocles (504 to 443 B.C.), who proposed the universe and everything in it was composed of four cosmic elements...