We’ve mostly forgotten how to think like our ancestors, which is why concepts like “heat and moisture” don’t make immediate sense to us.
But alongside modern medicine, interest in the four temperaments persisted. That’s how we end up with interesting cases like the 19th Century German-Jewish anatomist Jakob Henle, for whom the Loop of Henle in the kidney is named (and whose marriage to a maid and seamstress was the inspiration for Pygmalion, and thence My Fair Lady).
Henle was at the forefront of cell physiology using microscopes, became a proponent of the then-unpopular contagion theory of infection, and developed the four basic categories of tissue still used today.
Henle also wrote on temperament, and sought to explain the widely accepted four types in more up-to-date biological terms, specifically in terms of the nervous system.
When nerve cells receive a stimulus they become excited. Excitation in this sense simply means activity.
Henle believed that a person’s temperament was a reflection of the tonus of their nervous system: how easily excited the cells are, and how long they remain active or excited after the stimulus is removed.
Cholerics are excitable and form enduring impressions. This means they react strongly and quickly to stimuli, and their reaction lasts for a long time.
Sanguines are also excitable, but their impressions are comparatively short-lived, leaving them susceptible to distraction. They react strongly and quickly to one thing after another.
Melancholics are not very excitable. Our reaction to stimuli is comparatively slow and weak, but like the choleric our reactions last a long time.
Phlegmatics are not easily excited either, but unlike the melancholic their impressions are short-lived.
Each temperament’s way of seeing the world can be viewed as an outcome of these characteristics.
Why are cholerics “ambitious”? Because they have strong quick reactions to stimuli and these reactions last a long time. What we mean by ambition is strong desire that endures.
Why do sanguines like nice things and good experiences? Because they too react strongly and quickly to stimuli, but because their reactions are brief they are constantly drawn to new and exciting things.
Why are melancholics “idealists”? Because we aren’t excited enough by stimuli, so we are drawn to ideas that magnify the significance of everyday life. A new car doesn’t excite us much. But a new electric car is enhanced by ideals like environmentalism, game-changing technological advancement and breaking of tired conventions. Now that’s exciting! (And I don’t even own one).
Why are phlegmatics easy-going and rule-abiding? Because they have slow, weak reactions like the melancholic, but these reactions are brief like the sanguine. They aren’t strongly excited by anything, and they don’t dwell on things either. Following the rules is just the obvious thing to do, especially if it helps everyone get along and avoid conflict.