Hippocrates is generally credited with turning away from divine notions of medicine and using observation of the body as a basis for medical knowledge. Prayers and sacrifices to the gods did not hold a central place in his theories, but changes in diet, beneficial drugs, and keeping the body "in balance" were the key.
Central to his physiology and ideas on illness was the humoral theory of health, whereby the four bodily fluids, or humors, of blood, phlegm, yellow bile, and black bile needed to be kept in balance. Illness was caused when these fluids became out of balance, sometimes requiring the reduction in the body of a humor through bloodletting or purging.
The Hippocratic Corpus, or the collected writings attributed to Hippocrates, contains about sixty works on a variety of medical topics, including diagnosis, epidemics, obstetrics, pediatrics, nutrition, and surgery. There are assumed to be several authors, however, probably scattered over several centuries, and different treatises often give contradictory advice.
Hippocrates' followers wrote over 60 medical books on a broad range of medical topics (including diseases, gynecology, and head wounds). As to the Hippocratic Oath, little is known about who wrote it or first used it, but it appears to be more strongly influenced by followers of Pythagoras than Hippocrates and is often estimated to have been written in the 4th century BCE. Over the centuries, it has been rewritten often in order to suit the values of different cultures influenced by Greek medicine. See Hippocratic Oath.