Alcmaeon of Croton (a Greek city-state in southern Italy) was a pioneer in the study of human psychology and physiology. He published one book in the late sixth or first half of the fifth century BCE. Only two or three fragments of the book survive, but substantial reports of his views are preserved in authors such as Theophrastus. It is controversial whether Alcmaeon was primarily a physician and medical writer or whether he dealt with physiological issues as part of a typical pre-Socratic account of the cosmos. Beginning in the second century CE, some authors call him a Pythagorean, but the earliest sources do not. Aristotle appears to distinguish him from the Pythagoreans (Metaph. 986a22).

Alcmaeon is the earliest author to state the common ancient view that health depends on a balance of opposed powers in the body...