Hippocrates (460?–377? B.C.), a Greek physician called “the father of medicine.” Nothing reliable is known of his life, but it is said he studied at the temple of Asclepius (god of medicine) on the island of Cos. The Hippocratic Collection is a group of manuscripts attributed to Hippocrates, but most or all were written by his followers over a span of centuries. The Hippocratic Oath, sworn to by persons about to enter medical practice, is based on a code of ethical conduct appearing in the Hippocratic Collection.

The greatest achievement of the followers of Hippocrates was their emphasis on observation and their belief that all illness derives from natural causes. They emphasized the importance of rest and proper diet. However, they thought that the body is composed of four fluids called humors—blood, phlegm, black bile, and yellow bile—and that a change in the proper proportion causes illness. The theory of humors was adopted by Galen and accepted throughout the Middle Ages.