Hippocrates did it first: More than 2,000 years ago, the Greek physician theorized that four bodily fluids—blood, phlegm, black bile, and yellow bile—affect human traits and behaviors. He described four fundamental temperaments: sanguine (optimistic leader-like), choleric (bad-tempered or irritable), melancholic (analytical and quiet), and phlegmatic (relaxed and peaceful). Scientists long ago discredited his theory, but the practice of dividing the human personality into four quadrants lives on.