A test tube of blood left to stand overnight will settle into four layers. A black clot at the bottom, a layer of red blood cells, a white buffy coat and yellow serum at the top.
This finding may relate to the origin of one of the oldest theories of Medicine, the humoral theory. Ancient scholars of medicinal lore proposed that our bodies were composed of four humours: black bile (melan chole), blood, phlegm and yellow bile
These four humours were considered to be the basic units and fundamental building blocks of all nature. In good health it was beleived that our humours were in harmonious balance throughout the body. Illness was thought to be the manifestation of that balance being disturbed.
Our minds too were considered to be part of this dynamic equilibrium and the same four temperaments were assigned to specific personality and character traits. Hence our familiarity with the terms ‘melancholy’ or ‘sanguine’ personalities.