The document discusses the physiological and astrological beliefs about human temperaments in the Middle Ages. People were thought to have four humours - blood, phlegm, yellow bile, black bile - which determined one's complexion and personality. It then examines characters in Chaucer's Canterbury Tales through this lens, such as the choleric Summoner and melancholic Friar. It analyzes how their depicted personalities and vices align with the typical traits of their humour-defined complexions.