Dr. Semmelweis: he figured out that it was REALLY, REALLY DUMB to go straight from the anatomy lab to the delivery room without washing hands in between. He didn't understand why that was so -- Pasteur and the germ theory were most of a generation away -- but he saw what happened and changed his practice. For recommending something that saved women's lives, he was ridiculed. (Within 3 years he killed himself.)

Doctors need to be empiricists. You can know that something works before you know why it works.