The Six Non-Naturals: The Operating Manual of Humoral Medicine

Air, food, sleep, motion, evacuation, and the passions of the mind were the six things a physician told you to manage. They were the working core of humoral medicine, its everyday preventive plan.

In the early years of the fourteenth century, the physician Arnald of Villanova wrote out a set of instructions for James the Second, king of Aragon. The king was not ill. What Arnald produced was a regimen sanitatis, a plan for staying well: how to choose where to live, when and what to eat, how much to sleep, when to be bled, and how to keep his temper from spoiling his blood. Almost the whole book turned on six ordinary things.

Those six things had a name. Physicians called them the non-naturals, sex res non naturales, and the phrase misleads a modern ear. It did not mean unnatural. It meant the factors that are neither part of your fixed makeup nor a disease, but sit between the two: outside the body, met with every day, and largely within your power to change.

Naturals, non-naturals, and things against nature

Galen had divided the whole of medicine into three. First came the naturals, the body's own furniture: the elements, the four humors, the organs, and the faculties that drive them. Last came the things against nature, which meant disease and its causes. Between them sat a third class, the non-naturals: air, food and drink, sleep and waking, motion and rest, repletion and evacuation, and the movements of the mind. In the ninth century the translator Hunayn ibn Ishaq, known to Latin readers as Johannitius, set this scheme out plainly in a short introduction to Galen. When medieval schools began teaching medicine from a fixed shelf of texts, that little book stood near the front of it, and so every physician learned the same six.

The logic was preventive. Your humors could be pushed out of balance, but the push almost always came through one of these six doors, so a physician spent more time on regimen than on remedies. The list reads less like a theory than an operating manual for daily life.

Air, and the room you live in

The first non-natural was air, and it covered far more than breathing. It meant climate, wind, the site of your house, even the air of a crowded sickroom. The old Hippocratic essay Airs, Waters, Places had argued that a town facing hot southern winds bred one kind of body and one kind of illness, and a town open to cold northern air bred another. Physicians advising the rich told them which way their windows should face and how to sweeten a stale room with vinegar or burning herbs. Air was thought to enter and change the humors directly, so the air you kept was a medical decision.

What goes in, and what comes out

Two of the six governed the traffic through the body. The first was food and drink. Every item on the table had a nature, hot or cold, moist or dry, and the art of eating was to correct your own leaning with its opposite, a subject worked out in detail in Food and the Four Humors. The line between kitchen and pharmacy was thin, since many herbs were simply stronger foods used to steer the same balance.

The second was repletion and evacuation, the filling and emptying of the body. Too much blood, too much of any humor, was a load to be lightened. This is the reasoning behind bloodletting, purges, sweating, and the rest of the old treatments: not attacks on a germ, but ways of restoring a flow that had backed up.

Sleep, motion, and the passions of the mind

The last three were closer to home. Sleep and waking had to be measured, since sleep was thought to cook and settle the humors, while too much dulled them and too little dried the body out. Motion and rest meant exercise before meals to warm the body, then quiet afterward to let digestion finish. The sixth was the strangest to modern eyes and the most modern in effect: the passions of the mind, the accidents of the soul. Anger heated the blood, grief chilled and dried it, fear drove it inward, joy spread it out. A physician treated mood as a physical force, because in this system it was one. The verse regimen from Salerno put all of this within reach of anyone:

If doctors fail you, let three things be your doctors: a cheerful mind, rest, and a moderate diet.

Why the list outlived the theory

Here is the honest part. Take away the humors, the degrees of heat and cold, the bleeding by the calendar, and look at what remains. Clean air and a well-sited home. A sensible diet. Enough sleep, but not too much. Exercise, then rest. Regular elimination. A settled mind. That is close to word for word what a doctor tells you now, and it is why the framework held for more than a thousand years after the theory beneath it had begun to crack.

The non-naturals also explain a habit the temperaments still carry. A physician did not treat everyone alike. He first read your complexion, then set the six to suit it: more cooling food for a hot nature, more rest for a dry one. Your temperament was the starting point, and the regimen was the reply. That reading began with the same question the test asks today, before a word of advice about air or sleep or supper was written down.