Basics of the Four Temperaments
Back in the days of the Greeks, they did not understand the inner workings of the human body the way we do now. Therefore, they thought that a way a person acted was due to actual fluids in them. They called these fluids "humours".
People were said to have blood, yellow bile, black bile and phlegm in them. If one of these items was bloating your system (like too much blood for example) then it would affect your personality. A "normal" person had all four items in balance.
This theory was believed even through the middle ages.
Four Temperaments >
Personality Types >