The theory of the Humors began around 500 B.C. with Hippocrates whose observations about blood led to his noting how it separates into four parts:

1. the clear red
2. a yellowish liquid that rises to the top
3. the dark liquid that settles to the bottom
4. the whitish fluid

Hippocrates and his students, especially his son-in-law Polybus, employed these observations to develop a theory that good health emanates from a balance of the four liquids, or "humors."  This theory was later expanded upon by Galen.  And, from Galen's theory of humors there evolved a theory of the temperaments. This theory of temperaments --tempers in the blood--led to almost every aspect of comportment being informed by the theory of the humors.  Chaucer and Shakespeare, for instance, utilized this theory for characterization, including physical appearance, social position and profession, and behavior.   Hamlet, of course, is the Melancholic and Ophelia, "too much of water," is Phlegmatic. Also, in one scene in which Hamlet talks with Guildenstern, King Claudius is alluded to as being "distempered with choler."

Thus, the theory of temperaments led to the concept of four personality types:  Choleric, Sanguine, Melancholic, and Phlegmatic.  While people have a dominant trait of one of these four, there are also secondary and tertiary temperaments in them. Here are the characteristics of each of the temperaments, or Humors:

1. Choleric - the humor of Yellow Bile is associated with the speen and with Fire, the hot and dry element.  In a person who has this as a dominant trait, he/she can easily be quick to anger, aggressive, assertive and domineering.  Choleric people usually make good leaders, but they must control their fiery temperament.
2. Sanguine - the humor of Blood is associated with the liver and with Air, the hot and moist element.  Sanguine personalities are also quick to anger and aggression, but their emotional responses do not go as deeply as those of the Choleric personality.  Sanguine people are usually fun-loving, but they are also easily angered although this emotion quickly dissipates while in the Choleric it lasts.
3. Melancholic - the humor of Black Bile is associated with the gall bladder and with Earth, the cold and dry element.  Melancholic people tend to be moody and somewhat pessimistic or depressed as they perceive the negative aspects of life.  They can be worriers as they "carry the world on their shoulders," pondering the problems of life often. Melancholic people, however, are intense and deep, often of a sensitive and creative personality
4. Phlegmatic - the humor of Phlegm is associated with the lungs and brain and with Water, the cold and moist element. The Phlegmatic personality is passive, sluggish and often indolent. Phlegmatic people are not self-motivated and must be encouraged to work and be active.  On the positive end, their relaxed ways often make people comfortable around them. 

The first two, Choleric and Sanguine are considered the active temperaments, and Melancholic and Phlegmatic the passive temperaments.  Usually a person has at least one active and one passive in his/her personality.