Greek philosophers
Alcmaeon of Croton
Alcmaeon of Croton was a Greek philosopher and physician. He was close to Pythagorean thought, which he was the first to apply to medicine. He was also marginally interested in astronomy and mathematics.
Alcmaeon of Croton claimed that health is the result of a balance of opposites. And therefore, that illness signifies an imbalance between them. These basic opposites included cold-heat or wet-dry. He reached his conclusions not by mere deduction, but by practice. He carried out various operations, but also dissected animals.
Unfortunately, his life's work has not survived, although there are many references to him in ancient literature (Hippocrates, Plato).