The four humors are the basis of ancient medicine. Essentially, according to the four humors model, general health is considered to depend on the balance of four major body fluids: blood, phlegm, black bile, and yellow bile. The concept arose in ancient Greece, but persisted until the 19th century. Although the idea of the four humors and their effect on general health and temperament has been dismissed in the medical field, many modern theories of psychology are based on the four personality types associated with the four humors.
Ancient Greek and Roman thinkers and physicians theorized that physical and mental disorders were the result of an imbalance in one of the four humors. An excess of any of the four was thought to correspond to a certain temperament in the patient. A large amount of blood made the patient optimistic or happy, perhaps with too much energy. Too much phlegm did it phlegmatic , or cold and listless. An excess of black bile, also called spleen or blue bile and believed to be excreted by the spleen, would make a person melancholy or depressive. Finally, too much yellow bile, or anger, made a temper choleric or easily angry.
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 humors was believed to be hot or cold and dry or moist, each corresponding to one of four possible combinations of these attributes. To treat an excess of phlegm, then, which was considered warm and moist, the patient would be given food considered cold and dry. This system is the basis for today's food and wine classifications that use these terms, such as a "hot" pepper or a "dry" white wine.
The four humor system became a thing of the past with a more modern and accurate understanding 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 humors are still considered useful in psychology, where the four basic categories of human personality are considered, and personality disorders are grouped according to them.