The Hippocratic Oath was conceived by the ancient Greek physician Hippocrates, who is commonly referred to as the father of medicine. The oath is considered to be a cornerstone of medical ethics.
Hippocrates sought out common sense explanations for diseases, instead of ascribing supernatural causes. By the time he died in 377, he had become the most widely-known physician in Greece.
The classical oath is dedicated to the Greek gods, and explicitly bars abortion and euthanasia. Additionally, the classical oath prohibits doctors from performing surgery.