The Four Temperaments: Humoralism
Introduced by the Egyptians (possibly even by Mesopotamians), and codified by the ancient Greeks, the theories of Humourism (the term used in philosophy) and Humoralism (the term used in medicine) tied all personalities, illnesses, and strengths of a person to the balance of four basic fluids in the body: Black Bile, Yellow Bile, Phlegm, and Blood.
Humoralism was one of the most enduring (but now discredited) theories of medicine, lasting over 2000 years, and only truly being pushed to the side following the 1858 publication of Rudolph Virchow’s theories of cellular pathology. However, humorism, as used in philosophical theories and later, psychology, remained as the basis of the “Four Temperament" theories, and as a metaphorical descriptive device in psychology texts.
In Ancient Greece, Hippocrates first outlined the characteristics of each humour, and a couple hundred years later Galen codified the additions to Hippocrates’ theories. The Islamic physician Avicenna also contributed to the theories, and his additions (including the association of mental capacity and morality to each humour) later played a role in the development of the also-discredited field of phrenology.
The ideal person had a perfect balance (eucrasia - “good mixture”) of all four humours, but certain people were disposed to an overabundance of one or more, causing a dyscrasia, or “bad mixture”. This “bad mixture” caused disease and changes in appearance, and later was thought to cause changes in personality, as well.