Hence the clever choice of "humor" as a descriptive term for proposed modules of temperament-with the oldest and most venerable of gloriously wrong theories in the history of medicine. For more than a thousand years, from Galen to the dawn of modern medicine, prevailing concepts regarded the human personality as a balance among four humors-blood, phlegm, choler, and melancholy. Humor, from the Latin word for liquid (preserved in our designation of the fluids of the human eye as the aqueous and vitreous humors), referred to the four liquids that supposedly formed the chyle, or digested food in the intestine just before it entered the body for nourishment. Since the chyle arose, on one hand, from a range of choices in the food we eat and, on the other hand, from constitutional differences in how various bodies digest this food, the totality recorded both innate and external factors-an exact equivalent to the modern claim that both genes and environment influence our behavior.
The four humors of the chyle correspond to the four possible categories of a double dichotomy-that is, two axes of distinction based on warm-cold and wetdry. The warm and wet humor is blood; cold and wet generates phlegm; warm and dry makes choler; while cold and dry forms melancholy. I regard such a logically abstract scheme as a heuristic organizing device, much like Cloninger's quadripartite theory of personality. But we make a major error if we elevate such a scheme to claims for real and distinct physical entities inside the body.
In the medical theory of humors, good health results from a proper balance among the four, while distinctive personalities emerge from different proportions within the normal range. But too much of any one humor may lead to oddness or pathology. As a fascinating linguistic remnant, we still use the names of all four humors as adjectives for types of personality: sanguine-dominance of the hot-wet blood humor-for cheerful people; phlegmatic, for stolid folks dominated by the cold-wet humor of phlegm; choleric, for angry individuals saddled with too much hot-dry choler; and melancholic, for sad people overdosed with black bile, the cold-dry humor of melancholia.