Why did doctors believe for 2,000 years that your body ran on four fluids?

Because the idea was tidy, it seemed to explain everything, and for centuries doctors trusted authority more than they checked bodies. In ancient Greece, physicians taught that the body holds four fluids, called humours: blood, phlegm, yellow bile and black bile. Health meant having them in the right proportion and well mixed. Illness meant one of them had become deficient, in excess, or separated from the others. Because the imbalance of humours was thought to be the direct cause of all diseases, a doctor's job was to restore the balance.

The theory was easy to apply to people. Each humour came with a temperament: someone with too much blood was sanguine, and the very word melancholy comes from the Greek for black bile. The humours had seasons, too: blood was hot and wet, which tied it to spring. A doctor could link your mood and the season to your fluids without ever opening a body.

The treatment followed the same logic: get rid of the excess. That meant purging, bloodletting and other ways of draining the body. The theory owed much of its staying power to one man, Galen, a Greek physician working in Rome. Galen's views dominated Western medical science for more than 1,300 years.

Galen had a problem, though. Roman law banned dissecting human bodies, so he cut open animals, mostly primates, and assumed that humans were built the same way. Because his authority went unquestioned, his mistakes went unchecked for centuries. In all, humorism dominated Western medical thinking for more than 2,000 years before the discovery of microbes finally disproved it.