The Hippocratic Oath is the pledge taken by many medical students upon graduation or upon entering into practice. While the text of the oath varies by translation, one important line reads, “I will prescribe regiment for the good of my patients according to my ability and my judgment and never to harm anyone.” The vows are attributed to the Greek physician and teacher Hippocrates (c. 460–c. 377 B.C.), who practiced on the island of Cos. Unlike his predecessors, who relied on superstitious practices in their treatment of patients, Hippocrates believed that diseases were brought on not by supernatural causes but by natural ones. He further believed that disease could be studied and cured; this assertion forms the basis of modern medicine, which is why Hippocrates is called the “father of medicine.”

It is largely owing to another prominent Greek physician that the oath was handed down through history: Galen (A.D. 129–c. 199) was physician to Roman emperors Marcus Aurelius (121–80) from 161 and Lucius Commodus (161–92) from 168. He demonstrated that arteries carry blood, not air (as had been thought), and, like Hippocrates, Galen believed in the four humors of the body. He left medical texts that for centuries were considered the authoritative works on medical practice. Galen’s writings reveal his high regard for Hippocrates, who lived and worked many centuries earlier.