The idea of the four temperaments traces back to an Ancient Greek medical theory that there were four fundamental bodily humors (blood, yellow bile, black bile, and phlegm) which could cause illness if they were out of balance. The terms sanguine, choleric, melancholic and phlegmatic were coined by the Greek physician Aelius Galenus to describe the effect of these humors on human behavior. This idea is perhaps one of the oldest that looks like a theory of personality. The four temperaments have never been a part of modern medicine or psychological science, but remain fairly well known due to their use by self-help and spirituality authors.