Galen

Galen trained in his native Pergamon at the sanctuary of Asclepius, where the sick came seeking dream-cures and a young ambitious physician could also learn a great deal of practical anatomy from the wounds of the gladiators he was assigned to patch up. Roman law forbade dissecting human corpses, so he built his anatomical knowledge on pigs, apes, and other animals, extrapolating to humans with a confidence that turned out to be right astonishingly often and wrong in ways nobody would catch for over a thousand years. He moved to Rome and became personal physician to a string of emperors including Marcus Aurelius, treating the philosopher-emperor's chronic stomach troubles while presumably restraining himself from mentioning that he had also read the man's Stoic handbook. His surviving works, remarkably voluminous for antiquity, cover anatomy, physiology, pharmacology, and a systematic theory of the four humors he inherited from Hippocrates and elaborated into a complete account of health as balance and disease as imbalance. He insisted, against rival medical sects of his day, that reasoned theory and careful observation had to work together, and staged public dissections and demonstrations partly to settle arguments and partly, by his own evident enjoyment of the spectacle, to win them. His authority became so total in both the Christian and Islamic medical worlds that for over a millennium, disagreeing with Galen was often treated as a stronger argument against your own eyes than against him — a body of authority Ibn Sina absorbed and systematized further, and that the anatomist Andreas Vesalius would only begin dismantling in the sixteenth century, discovering that Galen had never actually dissected a human body at all.