The Medievals believed that the building blocks of the world were the 4 Contraries: heat, cold, moist, dry. In differing combinations, these make up the four elements of the material world:
hot+dry= fire; cold+dry=earth; hot+moist=air; cold+moist=water

Humans were seen as microcosmic reflections of the larger world (macrocosm). Their dispositions, characters, minds, morality, and temperament were governed by four fluids or humors that were analogous to the four elements. Humors released spirits or vapors in the body that affected the brain and so the person's behavior. According to the predominating humor, both the character and the appearance of the person were affected.

Hot+dry yellow bile choleric temp. Tall and lean; easily angered; vindictive; high strung

cold+dry black bile melancholic Lean, doesn't sleep well; opinionated; long and fretting anger; fearful dreams

hot+moist blood sanguine white and ruddy complexion; merry; peppery; plump; cheerful; hopeful

Cold+moist phlegm phlegmatic fat; white complexion, excess sleeping, slow, dull, cowardly