Galen lived in the Roman Empire during the late 100s AD. He is primarily known for his contributions to medicine, especially for his refinement of the Hippocratic theory of the four humors.
The four humors are blood, yellow bile, black bile, and phlegm; and their respective temperaments are sanguine, choleric, melancholic, and phlegmatic. A person whose dominant humor is blood will be courageous, hopeful, playful, and carefree. A person whose dominant humor is yellow bile will be ambitious, a leader, restless, and easily angered. A person whose dominant humor is black bile will be despondent, quiet, analytical, and serious. A person whose dominant humor is phlegm will be calm, thoughtful, patient, indifferent, and peaceful.
All diseases, according to the humorist theory, arise from an excess or deficiency in one or more of the humors. Therefore, bloodletting was performed in an attempt to balance the humors, and restore the patient to good health.
Anyone who is familiar with Shakespeare’s plays knows that the humor theory was still popular during the time of Shakespeare, which is a testament to the enduring influence of Galen, who wrote 1500 years earlier.
Galen also wrote extensively about the human circulatory system. He was the first person to make the distinction between venous and arterial blood.
“Since feeling and voluntary motion are peculiar to animals, whilst growth and nutrition are common to plants as well, we may look on the former as effects of the soul and the latter as effects of the nature.”