As a medical therapy, bleeding or blood-letting endured for approximately 2,500 years. It was only abandoned at the beginning of the 19th century. The roots of bleeding as a medical therapy can be found in the Aristotlean idea that all matter is composed of four elements: air, fire, earth, and water. The ancient Greek physician Hippocrates adopted this idea to explain health and disease in humans. In the body, the four elements were represented by four humors: blood, phlegm, black bile, and yellow bile. When the humors were ideally balanced, a person enjoyed good health; if the humors were unbalanced, a person suffered an illness. Unbalanced humors were supposed to be caused by an over-accumulation of one of the four humors. Quite reasonably, physicians decided that to regain balance, and therefore health, it was necessary to rid the body of the excess humors. Galen (A.D. 130-200) endorsed Hippocrates's...