Chart of the Four Humours
The theory of the four humours (temperaments) is likely the oldest personality theory in the world. It can be traced back to the 4th century BC Greek physician Hippocrates, who believed that human moods were affected by an excess of four bodily fluids (called humours): blood, yellow bile, black bile, and phlegm. In the 2nd century AD, another Greek physician, named Galen, took this idea and made it into a personality theory. He believed that the fluid you had most in excess made you either a sanguine, choleric, melancholy, or phlegmatic.
[E+] Extraverted / [N+] Neurotic
quick reaction + long/intense reaction
bold + task-oriented
active, excitable, aggressive, impulsive
[E+] Extraverted / [N-] Calm
quick reaction + short/mild reaction
bold + relationship-oriented
sociable, talkative, easy-going, carefree
[E-] Introverted / [N+] Neurotic
slow reaction + long/intense reaction
quiet + task-oriented
moody, rigid, pessimistic, quiet
[E-] Introverted / [N-] Calm
slow reaction + short/mild reaction
quiet + relationship-oriented
passive, even-tempered, controlled
Although temperament is not actually based on bodily fluids, it is probably genetic. In the 20th century, Hans Eysenck found that the four humours matched roughly to the four ways in which the traits Extraversion and Neuroticism can be combined (as shown to the right).
There are however some differences in how people distinguish the Cholerics and Melancholics from the Saguines and Phlegmatics. Some see the distinction mostly in terms of reactions (long & intense vs short & mild) and others see it primarily in terms of task-orientation vs relationship-orientation.
In fact, some of those in the latter camp actually replace the phlegmatic quadrant with a new, fifth temperament called the Supine, and move the Phlegmatic to the center as a neutral, ambiverted temperament.
Correlations with Myers-Briggs & Keirsey Temperaments
Some theorists have attempted to correlate the four humours with the Myers-Briggs-based Keirsey temperaments. However, if the four humours are indeed comparing the two traits of Extraversion and Neuroticism, this is not possible since the Myers-Briggs doesn't measure Neuroticism. Therefore one cannot go any further than to say that Cholerics and Sanguines are E's and Melancholics and Phlegmatics are I's.