Bloodletting was based on an ancient system of physiology called "humoral medicine." Originated by the Greeks, humoral medicine was based on the premise that the elements of the body reflected those of the natural world; those natural elements included air, water, earth, and fire. The Greeks believed that the interplay of those elements affected all the forces of the universe.
The analogous bodily humors comprised phlegm, yellow bile, black bile, and blood. Greek physicians maintained that good health depended on maintaining a balance of the humors. In order to restore health, doctors would drain "excess" humors by purging the digestive tract or draining blood. The linguistic remnants of the system of humors are with us today, in the words "bilious" and "phlegmatic," for example.
-- Douglas Starr
Photo: Courtesy of the National Library of Medicine.