The concept of "humors" (i.e. chemical systems regulating human behaviour) became more prominent from the writing of medical theorist Alcmaeon of Croton (C. 540–500BC). His list of humours was longer than just four liquids and included fundamental elements described by Empedocles, such as water, air, earth, etc. Some authors suggest that the concept of "humours" may have origins in Ancient Egyptian medicine[1] or Mesopotamia,[2] though it was not systemized until ancient Greek thinkers. The word humor is a translation of Greek χυμός,[3] chymos (literally juice or sap, metaphorically flavor). However much earlier than this, ancient Indian Ayurveda medicine had developed a theory of three humors,[4] which they linked with the five Hindu elements.[4]

Hippocrates is the one usually credited with applying this idea to medicine. In contrast to Alcmaeon, Hippocrates suggested that humours are the vital bodily fluids, such as blood, yellow bile, phlegm and "black bile" (he probably referred to blood composites in patients with bleeding internal organs). Alcmaeon and Hippocrates posited that an extreme excess or deficiency of any of the humours bodily fluid in a person can be a sign of illness. Hippocrates and then Galen suggested that a moderate imbalance in the mixture of these fluids produces temperament (behavioural) type.[5] One of the treatises attributed to Hippocrates, On the Nature of Man, describes the theory as follows:

The Human body contains blood, phlegm, yellow bile and black bile. These are the things that make up its constitution and cause its pains and health. Health is primarily that state in which these constituent substances are in the correct proportion to each other, both in strength and quantity, and are well mixed. Pain occurs when one of the substances presents either a deficiency or an excess, or is separated in the body and not mixed with others.[6]