https://en.wikipedia.org/wiki/Humorism

The four humors are black bile, yellow bile, phlegm, and blood.

4 humours: When blood is drawn in a glass container and left undisturbed for about an hour, four different layers can be seen. A dark clot forms at the bottom (the "black bile"). Above the clot is a layer of red blood cells (the "blood"). Above this is a whitish layer of white blood cells (the "phlegm"). The top layer is clear yellow serum (the "yellow bile").

closely related to the theory of the four elements: earth, fire, water and air; earth predominantly present in the black bile, fire in the yellow bile, water in the phlegm, and all four elements present in the blood.

These "humors" may have their roots in the appearance of a blood sedimentation test made in open air, which exhibits a dark clot at the bottom ("black bile"), a layer of unclotted erythrocytes ("blood"), a layer of white blood cells ("phlegm") and a layer of clear yellow serum ("yellow bile").

Blood

The blood was believed to be produced exclusively by the liver.^{[12][13]:103–105}

Yellow bile

Excess of yellow bile was thought to produce aggression, and excess anger reciprocally gave rise to liver derangement and imbalances in the humors.

Black bile

The word "melancholy" derives from Greek μέλαινα χολή (melaina kholé) meaning 'black bile', from the belief that an excess of black bile caused depression.

Phlegm

Phlegm was thought to be associated with apathetic behavior, as preserved in the word "phlegmatic" /flɛgˈmatɪk/.^[14]