On the Natural Faculties

Overview

On the Natural Faculties is about a hundred and ten pages, written in Greek around 170 CE by a physician from Pergamon who had trained at Alexandria, made his name treating gladiators, and ended up as doctor to the emperor Marcus Aurelius. Galen wrote an enormous amount, perhaps ten per cent of all surviving Greek literature, and this is one of the few pieces that works as a self-contained argument.

The subject is what makes a living thing alive and behave as it does. Galen’s answer is faculties: powers belonging to parts of the body, each responsible for a specific operation. It sounds circular to a modern reader, and Galen knows the objection and answers it, arguing that naming a faculty is a way of registering that something regular happens and of setting the question of how, not a way of closing it.

The tone is worth preparing for. Galen is brilliant, industrious, and phenomenally rude. Most of the book is an attack on the followers of Erasistratus and on the Atomists, particularly Asclepiades, and he does not merely refute them; he calls them lazy, ignorant and unwilling to look. The polemic is not decoration, since what he is defending is the claim that bodies have purposes at all.

The argument

Book I: what a faculty is, and the four of them. Galen begins by separating three kinds of change: growth, nutrition and generation, which belong to the nutritive soul and happen without our knowing, as against the motions of animals, which involve perception and will. Natural faculties are the powers behind the first group, and they operate in plants as much as in us.

He then names four, and they organise the rest of the book. The attractive faculty draws in what a part needs. The retentive holds it while it is worked on. The alterative changes it into the substance of the part. The expulsive gets rid of what is left. Digestion, for Galen, is not mechanical trituration but alteration, the stomach genuinely converting food into something else, and he attacks the idea that it is simply grinding.

The most contested of the four is attraction. Erasistratus had explained the movement of fluids by horror vacui, the refusal of nature to allow a void: matter follows matter because nothing can be left empty behind it. Galen’s objection is that this cannot explain selection. If it were mere suction, an organ would take whatever was nearest, and in fact the kidneys take urine, the gall bladder takes bile, and the stomach takes food. Something must be drawing what is proper to it, the way a lodestone draws iron rather than everything in the neighbourhood.

The kidney experiments. The best pages in the book are Galen’s demonstration that urine reaches the bladder through the ureters, against a school that held it passed as vapour through the tissues.

He describes the procedure so that a reader can repeat it. Open a living animal. Tie the ureters. Watch: the bladder stays empty and the ureters swell above the ligature. Untie them and the bladder fills. Now tie the penis instead, and the bladder distends. Press hard on the full bladder and nothing goes back up the ureters, because their entry to the bladder is oblique and acts as a valve. Each of these is an answer to a specific counter-argument, and the sequence is a genuinely fine piece of experimental reasoning.

Books II and III: the system at work. Galen applies the faculties to the whole economy of the body. Food is altered in the stomach, carried to the liver, and there turned into blood; the liver is the origin of the veins and of the nutritive blood, which seeps outward to be consumed by the tissues. The four humours are produced by the same process, each organ drawing off what suits it, the spleen taking black bile and the gall bladder yellow. Respiration cools the innate heat.

Throughout he defends teleology. Nature is a craftsman, nothing is made pointlessly, and the way to understand a structure is to ask what it is for. He is contemptuous of the view that the body is a chance arrangement of particles, and his argument is that such a view makes the observed regularities inexplicable and medicine impossible.

How to read it

Read Book I entire and the kidney chapters twice. Everything characteristic of Galen is there: the method, the polemic, the experiments and the doctrine.

Read it next to Harvey. De Motu Cordis is the book that brought this system down, and it is much more impressive once you see how coherent and how well-defended the thing it demolished was. Harvey did not beat a superstition; he beat this.

Watch the difference between the method and the conclusions. Galen tells you to cut, to tie, to watch, to repeat, and he designs experiments to discriminate between hypotheses. His physiology is nevertheless almost entirely wrong, because the blood does not seep outward to be consumed and the liver does not make it. Being empirical is not by itself enough, and this book is the clearest available demonstration of that.

Do not dismiss the faculties too quickly. Galen is registering that an organ does something specific and reliable, and marking the place where an explanation is needed. It is a placeholder, and a placeholder that survived fourteen centuries is worth understanding before it is mocked.

Read al-Razi afterwards if you can. His Doubts About Galen, written in Persia in the ninth century, is the best early demonstration that Galen could be admired and contradicted at the same time, which is more than most of Europe managed for another seven hundred years.

Questions it raises

• Is a faculty an explanation or a name for what needs explaining? Galen argues it is the second. Does his practice bear that out?
• Attraction is invoked because suction cannot account for selectivity. Is the argument sound, and what would a better answer have looked like in 170 CE?
• Galen designs experiments and gets the physiology wrong. What else is required beyond experiment?
• Nature does nothing in vain is offered as a research principle. Does assuming purpose help or mislead an investigator?
• Why did this system last fourteen hundred years? What features make a theory that durable, and are they the same features that make one true?
• Galen is savage about opponents who will not dissect. Is the contempt earned, and does it help the argument?