First, there is a strong tendency among people not to challenge conventional wisdom. As proof of that, I offer the fact that from AD 200 until roughly 1850, western medicine was based on the teachings of a physician named Galen. Galen did not know that blood circulated, or that the heart pumped it. He blamed disease on an imbalance among four bodily humors (black bile, yellow bile, water, and phlegm), and drew his conclusions about human anatomy from dissections of monkeys and pigs, not humans. But for nearly two millennia, Galen was more revered than Oprah.