On Generation and Corruption

Overview

On Generation and Corruption is sixty pages in two books, and it sits in a definite place in Aristotle’s natural philosophy. The Physics gives the general account of change, the treatise On the Heavens deals with the eternal and unchanging region above the moon, and this one takes the region below it, where things come into being and pass away. The Meteorology follows, and then the biological works.

The subject is what would now be called chemistry, though nothing in it is quantitative and no substance is isolated. The questions are conceptual: what distinguishes a thing’s coming to be from its merely changing, what it takes for two substances genuinely to mix, whether matter is continuous or made of indivisible particles, and where the elements come from.

It is dense going, more so than its length suggests, because Aristotle is arguing against several schools at once, mostly the atomists and Empedocles, and he assumes you know the Physics.

The Latin title, De Generatione et Corruptione, is the one it is usually cited by, and the Greek can also be translated as On Coming to Be and Passing Away.

The argument

Book I: kinds of change. The opening problem is a distinction. When something changes, sometimes a subject persists and takes on a new quality, and that is alteration: the same man becomes pale. Sometimes nothing persists at the level of the substance, and a thing ceases to exist while another begins, which is generation proper. Aristotle’s difficulty is that these are hard to tell apart in practice and that the earlier physicists ran them together. Those who make everything out of one underlying stuff, he argues, can only have alteration; those who make it out of many, only combination.

He then turns on the atomists. Democritus and Leucippus are given a respectful hearing, and Aristotle says explicitly that they thought more systematically about these questions than anyone else. His objection is about mixture. If bodies are indivisible particles in a void, then when two substances are combined the particles simply lie next to each other, and what we call a mixture is a mosaic too fine for the eye. But that is not what mixture is. When tin and copper make bronze, or wine is mixed with water, the result is uniform all the way down and has properties neither ingredient had, and you cannot recover either by looking closer.

His own account is in chapter 10 and it is subtle. In a genuine mixture the ingredients are altered by one another and survive in potentiality rather than in actuality: they are not there as themselves, and they are recoverable, which is why the mixture is a mixture and not a new simple substance. For this to happen the ingredients must be comparable in power, since a drop of wine in ten thousand gallons of water is not mixed with it but destroyed.

Book I also treats contact, action and passion. Things act on one another only if they are of the same genus and contrary in species, which is why fire heats water and one whiteness does not act on another.

Book II: the elements and the cycle. Aristotle now derives the four elements rather than postulating them. He starts from the tangible contrarieties, and argues that the fundamental ones are hot and cold, wet and dry, because the others reduce to these. Four qualities taken two at a time give six pairs, of which two are impossible, since nothing is both hot and cold. That leaves four: hot and dry is fire, hot and wet is air, cold and wet is water, cold and dry is earth.

The point of the derivation is transformation. Because each element is a pairing, any element can become any other by exchanging one quality or both, and the easiest changes are between elements that share a quality. Matter is one, and the elements are its four basic configurations. This is what makes the theory a chemistry rather than a list.

He then argues that coming to be must be everlasting and cyclical. If generation is continuous, something must keep driving it, and the cause cannot be the outer heaven’s uniform daily rotation, because a uniform cause produces a uniform effect. It must be a motion that approaches and retreats, and there is one: the sun’s annual travel along the ecliptic. When it comes near, things are generated; when it withdraws, they pass away. That is why the processes are balanced and why the natural world runs in cycles rather than toward an end.

The book closes by observing that in things which cannot persist as individuals, nature secures the next best thing, persistence of the kind, which is why generation is continuous.

How to read it

Read the Physics first, at least Books I and II. This treatise assumes matter and form, potentiality and actuality, and the four causes, and will not make sense without them.

Read Book I chapters 1, 2 and 10, and Book II chapters 1 to 4 and 10. That is the argument about change, the attack on atomism, the account of mixture, the derivation of the elements and the cosmic cycle.

Follow the mixture argument carefully, because it is the best thing here. The claim that ingredients persist potentially and not actually is Aristotle trying to describe something we would now call a compound, and it is a genuinely difficult position that neither atomism nor simple fusion can state.

Read it as the theory Lavoisier had to defeat. Two thousand years later, the assumption that substances are combinations of qualities and can be transmuted into one another by rearranging them was still the working framework, and the reason phlogiston seemed reasonable. What replaced it was not a better argument but a balance.

Do not dismiss the derivation of the elements as primitive. It is an attempt to explain why there are four rather than to assert it, and the ambition, to derive the kinds of matter from something more basic, is the same ambition that produced the periodic table.

Questions it raises

• What distinguishes a thing’s ceasing to exist from its merely changing? Aristotle needs the distinction and it is not obvious how to apply it to any actual case.
• The argument against atomism is that atoms cannot genuinely mix. Is that decisive, and how would a modern chemist answer it?
• Ingredients in a mixture are said to survive potentially. Is that an explanation or a way of naming the puzzle?
• Four elements derived from two contrarieties: what makes hot, cold, wet and dry the fundamental qualities rather than any other pair?
• Generation is continuous because the sun approaches and retreats. Is that a causal explanation or an observed correlation dressed as one?
• The system lasted two thousand years and is entirely wrong. What kept it in place, and what would have had to be different for it to fall earlier?