What was the role of the four humours according to Galen in understanding personal character and temperament?

They were essential body fluids responsible for different temperaments.

Galen elaborated on the role of the four humours in personal character and temperament. According to him, the material world was made up of four elements, viz. earth, air, fire, and water which combined to form four essential body fluids, viz. blood, black bile, yellow bile, and phlegm. Each of these fluids was seen to be responsible for a different temperament. Imbalances among the humours were believed to cause various disorders.