古代・中世ヨーロッパの医学・病理学においては，心身の病態を体液 (humours) に基づいて説明する原理が広く受け入れられていた．とりわけ古代ギリシアの時代よりヒッポクラテスやガレノスが指摘してきた4種の体液 (the four cardinal humours) が重要な役割を果たすとされた．血液 (blood)，粘液 (phlegm)，（黄）胆汁 (choler，yellow bile)，黒胆汁 (melancholy, black bile) の4種である．これらのバランスがよいと健康となるが，不足や過剰が生じると，支配的な体液の種類に応じて特有の病態，気質，気分が現出するという．Crystal and Crystal (230) の説明を引こう．
In early account of human physiology, a person's physical and mental disposition was thought to be governed by a combination of fluids, or humours, within the body. Four humours were recognized: blood, phlegm, choler (also called yellow bile), and melancholy (also called black bile or black choler). The notion transferred readily into a range of seances to do with temperament, mood, inclination, and manner of action . . . , regarded as permanent or alterable features of behaviour.
ルネサンス以降には解剖学が進歩し4体液説は徐々に廃れていったが，Shakespeare の時代にはまだ後者の影響が色濃く残っていた．Shakespeare からの例の一部を Crystal and Crystal (230) より挙げつつ，登場人物のなかに4体液説の具体的な反映をみてみよう．
| Humour | Typical disposition | Seen in character | Example |
| blood | optimistic, passionate, amorous, courageous | Hotspur (as described by his wife) | In military rules, humours of blood, / He was the mark and glass, copy and book, / That fashioned others (2H4 ll.iii.30) |
| phlegm | dull, indifferent, indolent, apathetic, idle | Falstaff and his companions (as described by Prince Hal) | I know you all, and will awhile uphold / The unyoked humour of your idleness, (1H4 l.ii.194) |
| choler | angry, irascible, bad-tempered | Cassius (as described by Brutus) | Go show your slaves how choleric you are ... Must I stand and crouch / Under your testy humour? (JC IV.iii.43) |
| melancholy | sad, gloomy, sullen, depressed | Jaques (as described by Rosalind) | They say you are a melancholy fellow. [Jaques] I am so: I do love it better than laughing (AY IV.i.3) |
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