An ancient medical theory originated by Hippocrates held that the human body was filled with four basic substances, humours, which are in balance when a person is healthy. All diseases and disabilities resulted from an excess or deficit of one of these four humors. The four humors were identified as black bile, yellow bile, phlegm, and blood. Greeks and Romans, and the later Western European medical establishments that adopted and adapted classical medical philosophy, believed that each of these humors would wax and wane in the body, depending on diet and activity. When a patient was suffering from a surplus or imbalance of one fluid, then his or her personality and physical health would be affected.
The concept of four humors may have origins in ancient Egypt or Mesopotamia, but it was not systemized until ancient Greek thinkers around 400 BC who directly linked it with the popular theory of the four elements earth, fire, water and air (Empedocles). Paired qualities were associated with each humour and its season. The word humour derives from the Greek ??µ??, chymos (literally juice or sap, metaphorically flavor).
Theophrastus and others developed a set of psychological characters based on the humors. Those with too much blood were sanguine. Those with too much phlegm were phlegmatic. Those with too much yellow bile were choleric, and those with too much black bile were melancholic. The idea of human personality based on humors contributed to the character comedies of Menander and, later, Plautus.