In the renaissance, a very important theory in medicine was the Four humours or Four temperaments. This theory had begun with the Ancient Greeks, and influenced medical development until the 19th century. According to the theory, the body was filled with four different substances, called "humours". When a person is healthy, these substances are in balance. When there is an imbalance between the humours, the person will be sick. The sickness can be cured by bringing the four humours back to balance. The four substances are: black bile, yellow bile, phlegm and blood. The Roman medic Galen still supported that theory, but refined it in many ways. 