Some modern ideas about the heart stem from the beliefs of ancient Greek scientists, known as philosophers. They came up with theories about how the human body worked—many of which turned out to be incorrect. “Ancient Greek philosophers believed that the human body was controlled by four humors, or fluids: black bile, yellow bile, phlegm, and blood,” explains Brad Bouley. He’s a professor of scientific and religious history at the University of California, Santa Barbara.
To the ancient Greeks, a healthy person was someone who had all their humors in balance. They believed that too much or too little of any of these fluids caused physical, emotional, or psychological problems. Too much yellow bile was thought to make a person angry, while too much black bile made them depressed. Blood was the humor associated with love and passion. “The heart was seen as a motor that heated fresh blood and moved it through the body, so it was also viewed as the origin of passionate feelings,” Bouley explains.