Parts of Animals

Overview

Parts of Animals is about a hundred and fifteen pages in four books, and it is one piece of a very large project. Aristotle wrote more on animals than on anything else, perhaps a quarter of everything that survives from him: the History of Animals gathers the observations, this work explains the structures, the Generation of Animals explains reproduction, and there are shorter works on movement and locomotion. Much of the observation was done on Lesbos, in the lagoon at Pyrrha, during the years after he left Plato’s Academy.

The book divides sharply. Book I is methodology and is famous. Books II to IV are the biology, working through the parts of animals in order, and are for readers with a specific interest.

The quality of the observation is the thing that surprises people. Aristotle dissected extensively, and he gets things right that nobody confirmed for centuries: that dolphins and whales are live-bearers with lungs and not fish, the chambered stomach of ruminants, the hectocotylus arm of the octopus, which is used in mating and which zoologists dismissed as a fable until it was rediscovered in the nineteenth century. He also gets a great deal wrong, including the function of the brain, which he thought cooled the blood.

The argument

Book I: how to do biology. Aristotle opens by asking whether a naturalist should proceed like a critic, first establishing the standards by which an account is judged, and answers yes, and then supplies them.

The first is about causes. A natural thing has four, and in the study of living things they do not carry equal weight. What a part is made of matters, but the material exists for the sake of the function, so the account must begin from what the part does. Empedocles and the older physicists tried to explain the backbone by saying it was broken into vertebrae by the twisting of the embryo, and Aristotle regards that as a category mistake: the question is why an animal with a spine has one segmented, and the answer is that it must bend.

From this follows the homonymy principle, which runs through all his biology. A hand that cannot grasp, a hand of bronze or a dead hand, is a hand only in name, because a part just is its function. A corpse has the same shape as a man and is not one.

The second methodological argument is against division by dichotomy. Plato’s Sophist and Statesman classify by splitting a genus in two at each step, and Aristotle objects that nature does not divide in pairs, that the method forces a single differentia to do all the work, and that it separates animals that plainly belong together, since birds and fishes would end up in different branches of a split based on having feet. He proposes instead grouping by many differentiae at once, working from the great kinds that ordinary observation already recognises.

Then chapter 5, the defence of the subject. Some animals are unattractive to look at and the study of them gives immense pleasure to anyone who can grasp causes. Aristotle grants that the heavenly bodies are nobler objects, and points out that we know almost nothing about them, whereas living things are here and can be examined thoroughly. He tells the story of Heraclitus, found warming himself at a kitchen stove, who told embarrassed visitors to come in because there are gods even in a kitchen, and says that we should approach every kind of animal without disgust, since in all of them there is something natural and something beautiful.

Books II to IV: the parts. The order is from simple to complex. First the uniform parts, those whose pieces are the same as the whole, blood, fat, marrow, brain, flesh, bone. Then the non-uniform parts built out of them, the organs and limbs.

Blood gets a long treatment, and Aristotle’s distinction between animals with blood and without is essentially our distinction between vertebrates and invertebrates. The heart is the seat of life and of sensation, the first part to form and the last to die, and the brain’s job is to cool it, which is wrong and is argued for carefully. The lung exists to cool as well. Teeth, horns, beaks, stomachs and limbs are each explained by what the animal’s way of life requires, and where a structure appears useless Aristotle either finds a function or attributes it to necessity rather than purpose, which is a distinction he keeps clear.

Book IV ends with the parts of bloodless animals and with locomotion, and with the observation that man alone stands upright, which he connects to the proportions of the body and to the fact that the human hand is free.

How to read it

Read Book I entire and stop there unless the zoology interests you. It is forty pages and it is one of the founding documents of the philosophy of science.

Read chapter 5 twice. The defence of studying humble creatures is the best short answer ever given to the question of why anyone would spend a life on worms.

Do not read the functional explanations as a religious argument. Aristotle’s nature is not a designer with intentions; the end is internal to the organism, which develops toward what it is to be. That is why the argument survives in modern biology as functional explanation, and why teleology in this sense is not the same as the argument from design.

Read it against Darwin. The obvious objection is that Aristotle’s purposes have no mechanism behind them, and natural selection supplies one. The less obvious point is that biologists still explain parts by what they are for, and that the question Aristotle asked turned out to be the right question with the wrong account of why it has an answer.

Keep his errors in proportion. He thought the brain cooled the blood and the heart did the thinking. He also identified the dolphin as a mammal, described the development of a chick embryo by opening eggs on successive days, and reported the octopus’s mating arm accurately enough that it could be recognised two thousand years later.

Questions it raises

• Can a part be explained without saying what it is for? Aristotle says no. Does modern biology agree, and does it mean the same thing by the question?
• A hand that cannot grasp is a hand in name only. Is that a discovery about hands or a decision about words?
• Nature does nothing in vain is used as a research rule. When does that rule help an investigator and when does it lead him to invent functions?
• Aristotle rejects dichotomous division because natural kinds do not come in pairs. What should a classification track, and how would you know?
• He got the heart and brain exactly backwards while getting the dolphin right. What distinguishes the questions he answered well from the ones he did not?
• If purposes in nature need no designer, what exactly grounds them?