Without a proper methodology, reason alone can lead you down a lot of blind alleys, so it comes as little surprise that the father of Western medicine also sired his share of quack ideas.
For example, Hippocrates sought natural causes for supposedly supernatural ailments, including the "sacred disease" of epilepsy -- then viewed as evidence of possession by gods or demons. He also pioneered the wrongheaded notion of bodily fluids, or humors, which he said determined human health, appearance and disposition. Medical practice based on balancing blood, phlegm, bile (also called choler) and black bile (aka melancholy), each purportedly regulated by a different organ, persisted until the mid-17th century. Its legacy lives on in words like sanguine (Latin sanguineus "of blood," meaning optimistic or positive) and melancholy (depressed) [sources: Encyclopaedia Britannica; NLM].
Physicians tried to regulate humors through diet and exercise, and by studying bodily evacuations like urine. So far, so good. The problem was, they reduced every ailment to these causes, mistreating or ignoring the roots of painful and deadly disorders for centuries. Indeed, far from abandoning the flawed fluids, practitioners doubled down on them, gradually tying humors to qualities (wet/dry, hot/cold), elements (earth, air, fire and water), seasons and stages of life. Similar ideas persist today in Indian Ayurveda and traditional Chinese medicine [sources: NLM; Science Museum (UK)].