The four humours are the basis of ancient medicine. Essentially, according to the four humours model, general health is held to be reliant on the balance of four major body fluids: blood, phlegm, black bile, and yellow bile. The concept arose in Ancient Greece, but persisted into the 19th century. Though the idea of the four humours and their effect on general health and temperament has been discarded in the field of medicine, many modern theories of psychology are based upon the four personality types associated with the four humours.
Ancient Greek and Roman thinkers and physicians theorized that physical and mental disorders were the result of an imbalance in one of the four humours. An excess of any of the four was thought to correspond a certain temperament in the patient. A large quantity of blood made the patient sanguine or cheerful, perhaps with too much energy. Too much phlegm made him or her phlegmatic, or cool and apathetic. An excess of black bile, also called spleen or melancholy and thought to be excreted by the spleen, would make a person melancholic or depressive. Finally, too much yellow bile, or choler, made for a choleric or easily angered temperament.
Medical treatments in the past were often attempts to rebalance the four humours. Bloodletting was common in the medieval era, and in the Elizabethan period, certain foods were thought to address complaints caused by an excess or deficit of certain humours. Each of the four humours was believed to be either hot or cold and either dry or wet, so that each corresponded to one of the four possible combinations of these attributes. To treat an excess of phlegm, then, which was considered warm and wet, the patient would be given foods considered cold and dry. This system is the basis behind current classifications of foods and wines using these terms, such as a "hot" pepper or a "dry" white wine.
The four humours system became a thing of the past with more modern and accurate understandings of human physiology. For example, it is now known that there is no such thing as "black bile" secreted by the spleen. However, the four temperaments associated with the humours are still considered useful in psychology, where they are considered the four basic categories of human personality, and personality disorders are grouped according to them.