Physics

Overview

These are lecture notes, not a book. Aristotle’s surviving works were his teaching materials, and the Physics reads accordingly: compressed, elliptical, full of back references and sudden restarts, and occasionally repeating an argument in two incompatible forms because both were tried in class. Sentences frequently omit the subject. Several chapters are catalogues of the ways a word can be used. Reading it is more like reconstructing a lecture than following prose.

The subject is not physics in the modern sense. Aristotle is asking what has to be true of the world for change to be intelligible at all, and he treats that as a question prior to any study of particular changes. He works out the framework here, and the specialised treatises, On the Heavens, On Generation and Corruption, the Meteorology and the biological works, use it. There is very little measurement and no mathematics of the kind Archimedes would use a century later. Aristotle thinks mathematics abstracts from change and so cannot be the science of changing things.

Two vocabulary points before you start. Kinesis, usually translated motion, covers every kind of change, including growth, alteration and coming to be, not just travel from place to place. And physis, nature, names an internal principle in a thing, not the collective world outdoors.

The argument

Book 1 clears the ground. Aristotle takes seriously the Parmenidean claim that change is impossible, since what is cannot become and what is not cannot do anything. His solution is to distinguish three things involved in any change: a substratum that persists, a form it acquires, and the lack of that form it had before. An uneducated man becomes educated. The man persists, and the objection dissolves because the thing that comes to be does not come from nothing but from something that lacked it.

Book 2 is the one most often assigned alone. It defines nature, distinguishes natural things from artefacts, and sets out the four causes: the material, the form, the source of the change, and that for the sake of which. Aristotle then argues at length that the fourth kind of cause operates in nature and not only in craft, against the view he attributes to Empedocles that organisms survive by accident. His evidence is regularity: teeth come in the same arrangement every time, and chance does not produce the same result repeatedly. Chance and luck get their own treatment as real but derivative.

Book 3 defines motion in terms of potentiality being brought to completion, and then takes up infinity. Aristotle denies that anything is actually infinite, whether in size or in number, but allows the infinite potentially: any magnitude can always be divided again, and any count can always be continued. This solution serves him for the rest of his career.

Book 4 treats place, void and time. Place is the inner limit of the surrounding body, which is why he can deny that the universe as a whole is anywhere. Void does not exist, and the arguments against it are among his most interesting, since they include an early account of why bodies would fall through a vacuum at speeds that make no sense. Time is the measure of change, and he asks pointedly whether it could exist if there were no soul to count.

Books 5 and 6 are technical. Book 5 classifies kinds of change and what can change into what. Book 6 handles continuity and divisibility, and answers Zeno: Achilles catches the tortoise because a finite time is divisible in exactly the way a finite distance is, and the arrow is not at rest at an instant because motion and rest are both predicated over intervals.

Books 7 and 8 drive to the conclusion. Everything moved is moved by something. A chain of moved movers cannot be infinite. Motion as a whole cannot have begun, since a first motion would need a cause and there would be no time before it. Therefore there is an eternal first mover, itself unmoved, and the motion it causes must be the continuous circular motion of the heavens, because only circular motion can go on forever without reversal.

How to read it

Read Book 2 first, complete. It is the most self-contained and the most influential part, and it gives you the four causes, which everything else assumes.

Then read Book 1, Book 3 chapters 1 to 3, Book 4 chapters 10 to 14 on time, and Book 8 chapters 1 to 6. Books 5 and 6 can wait unless you care about Zeno, in which case Book 6 is the reason to be here.

Read with a pencil and paraphrase each argument in your own words as you go. Aristotle’s text will not give you a second pass at an idea, and a paragraph you skim is usually a premise you will need forty pages later.

The standard misreading is to treat the four causes as four competing theories, one of which modern science picked. Aristotle thinks a complete explanation gives all four, and that they answer different questions about the same thing.

Do not judge the physics by whether it predicts. Aristotle is not trying to predict. He is trying to say what a thing is and why it behaves as its kind behaves, and his criteria for success are correspondingly different from Newton’s.

Questions it raises

• Is there any explanation left in nature that answers the question what for, or did biology only appear to bring it back?
• Aristotle asks whether time could exist with nothing to count it. What would it mean for the answer to be no?
• If place is the limit of a surrounding body, where is the universe?
• Why should a chain of causes not be infinite? Is the refusal a discovery or an assumption?
• Does defining motion through potentiality explain anything, or does it only rename what needed explaining?
• Aristotle’s arguments are almost all from what appears to be the case. When appearances and measurement conflict, which should give way?