The four humors theory dominated the medical field for thousands of years. It was based upon the concept that the human body was composed of four substances or humors: blood, yellow bile, black bile, and phlegm. Any sort of sickness was an excess or deficiency of those humors. For instance bloodletting was removing the extraneous blood to return to the proper amount. And the concept expanded to almost all aspects of a person's life: to this day someone who is phlegmatic means they are calm and rational, which meant they had an excess of phlegm in their body. Each food added or subtracted from a person's humor: A person who had too much black bile should avoid carp but eat pike. People with lots of yellow bile have curly hair.

The idea is attributed to Hippocrates, the father of the Hippocratic Oath, who was a famous Greek physician around about the 400s B.C. Later on in the 100s A.D. another prominent Roman physician named Galen popularized the idea, and through the Middle Ages and the Renaissance his works were practically the Bible for doctors throughout western Europe.

Beginning in the Renaissance, chinks began to appear in the four humors' armor. First a Belgian anatomist Vesalius in the 1500s began to notice Galen made several errors; Galen himself mostly performed experiments on animals, not humans. While that's good to realize things like nerves exist, it causes problems when trying to describe a human body. Then an Englishman named William Harvey in the 1600s discovered the circulation of blood. This actually completely smashed the theory entirely because it stated the four humors just lay still, but with Harvey's discovery the organs seem like mechanisms. But it wasn't until the 1800s that doctors let go of the four humors. Even though certain aspects were demonstrably false, only when a new theory — germ theory — arose then doctors could let go. Disease, instead of being an imbalance of say phlegm and yellow bile, could be seen as an infection.

I've known about this for quite some time, but I still hold my initial when I first came across this in middle school: What the FUCK was black bile? Blood and phlegm? Okay, can see that. I wouldn't really think that phlegm is good for you, but I'll roll with it. Yellow bile? Maybe they were confused with urine. I don't know. But what the fuck did they think was black bile? When Galen was dissecting his pigs, what did he find in there that made him think, "...Huh. Hippocrates was right. I see the black bile right there." People were killed constantly in the Middle Ages. Surely during all that stabbing someone must've come across an eviscerated body and thought, "I don't see any black bile anywhere." Did it seriously take people thousands of years to realize this shit doesn't make any sense?

I remember once in my medieval manuscripts class our teacher showed us an anatomy book of male and female genitalia. While the male portion was heavily detailed, the female side was just a triangle. I thought, "Wow, even back then guys had no idea how to deal with a uterus."