humour, (from Latin “liquid,” or “fluid”), in early Western physiological theory, one of the four fluids of the body that were thought to determine a person’s temperament and features. In the ancient physiological theory still current in the European Middle Ages and later, the four cardinal humours were blood, phlegm, choler (yellow bile), and melancholy (black bile); the variant mixtures of these humours in different persons determined their “complexions,” or “temperaments,” their physical and mental qualities, and their dispositions. The ideal person had the ideally proportioned mixture of the four; a predominance of one produced a person who was sanguine (Latin sanguis, “blood”), phlegmatic, choleric, or melancholic. Each complexion had specific characteristics, and the words carried much weight that they have since lost: e.g., the choleric man was not only quick to anger but also yellow-faced, lean, hairy, proud, ambitious, revengeful, and shrewd. By extension, “humour” in the 16th century came to denote an unbalanced mental condition, a mood or unreasonable caprice, or a fixed folly or vice.