The four humors are black bile, yellow bile, phlegm, and blood. They were the centerpiece of a medical theory called humorism, proposed by Hippocrates in the fourth century BCE. Until the 19th century, humorism dominated medical practice, with medical professionals ascribing most health problems to an imbalance in these fluids. The humors also appeared in Muslim medicine, spreading East to India, where they are still utilized in some traditional medicine. Modern physicians and medical researchers have firmly debunked the theory, although it has left some legacies behind.
According to the theory, the four humors were fluids that circulated in the body. An imbalance, caused by an excessive amount or a deficit of one or more humors, would cause medical problems. The humors were in turn associated with the four elements, and with heat, cold, moisture, and dryness. Using the complex system, physicians were supposed to be able to figure out where the problem was in their patients, and then they could use a variety of tools to correct the imbalance.
Someone with an excess of black bile was said to be melancholic, while someone with too much yellow bile was choleric. Phlegmatic people, as you might imagine, had too much phlegm in their systems, while sanguine individuals had an excess of blood.
The correction of imbalances could be achieved by eating specific foods that were designed to promote production of one humor or another. Patients could also be bled or fed emetics that caused them to vomit, thereby in theory restoring their balance of humors. Physicians also used heat, cold, moisture, and dryness to treat imbalances, which explains things like hot plasters, cupping, and other esoteric medical practices.
While it might seem strange to Westerners who believe in modern medicine, the four humors were taken very seriously until the 1800s, when people began to learn more about the mechanism of disease. The huge leaps and bounds made in the sciences in the 1800s essentially disproved this theory, however, putting a stop to more than 2,000 years of medical practice.
While the humors themselves have been disproved, this theory of medicine does have a lot of similarity with traditional medicinal practices in other cultures, especially Asia. Traditional medicine often suggests that health problems are caused by imbalances in the body that must be corrected, and some traditional medicinal treatments are quite successful, suggesting that there may be some truth to this idea.