The four temperament theory is a proto-psychological theory which suggests that there are four fundamental personality types: sanguine, choleric, melancholic, and phlegmatic. Most formulations include the possibility of mixtures among the types where an individual's personality types overlap and they share two or more temperaments. Greek physician Hippocrates described the four temperaments as part of the ancient medical concept of humourism, that four bodily fluids affect human personality traits and behaviours. Modern medical science does not define a fixed relationship between internal secretions and personality, although some psychological personality type systems use categories similar to the Greek temperaments.