Dreams mystified the ancient Greeks and Romans even more than they puzzle us, and both civilizations’ greatest minds attributed all manner of meanings to them: prophetic, divine, and even medical. (And you thought asking ChatGPT for medical advice was questionable.) 

Dreams were used as diagnostic tools by ancient physicians, including one of the most influential doctors in history: the Greek physician Galen, who wrote about the practice in a text called On Diagnosis in Dreams in the second century CE. He and others of his time believed that health was governed largely by the four humors — blood, phlegm, yellow bile, and black bile — and that too much or too little of any one humor resulted in illness. 

According to the theory, a person’s dreams corresponded with their ailments. “Someone dreaming a conflagration is troubled by yellow bile, but if he dreams of smoke, or mist, or deep darkness, by black bile,” Galen wrote in On Diagnosis in Dreams. “Rainstorm indicates that cold moisture abounds; snow, ice, and hail, cold phlegm.” The idea that dreams could be used in this diagnostic manner was a controversial one even then, and Galen wrote his text in defense of the practice. Humorism fell out of favor by the mid-19th century, when the development (and, in the following decades, acceptance) of the germ theory of disease changed medicine forever.