Cursus Philosophicus Thomisticus, Secunda Pars Philosophiae Naturalis

On the origin and passing away of things in general

Summary of the text of Aristotle’s first book On Generation and Corruption

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Aristotle’s purpose in this first book is to discuss the origin and passing away of things in general, and what follows or accompanies generation itself. In the second book, however, he treats what is generable and corruptible in particular, namely, the elements, which are the first generable bodies, and the mixed things compounded from them.

He divides his treatment of generation in general into ten chapters in this order. In the first two he reports the opinions of the ancients concerning generation and corruption and their distinction from alteration. In the next two he explains his own opinion. From chapter 5 through chapter 10 he treats what accompanies or follows generation, such as increase and nutrition, action and reaction, contact, and mixture.

Summary of chapter 1

In this chapter the Philosopher intends to refute two opinions of the ancients concerning the difference between generation and alteration; in the next chapter he rejects two others concerning the way in which some said generation occurs. The opinions of the ancients that he refutes in this first chapter are two:

The first was held by those who posited only one principle of natural things, and said that it was a being in act. They could not distinguish generation from alteration.

The second opinion belonged to those who posited several principles. They could posit substantial generation as distinct from alteration, but in fact did not posit it, because they said that the very qualities were the forms of the elements, such as heat and cold. They denied that one element could be converted into another, because they did not posit that the elements were composed of matter and form, but held them to be first matters not resolvable into other principles. In particular, Aristotle argues against Empedocles that he contradicted himself: on the one hand, he denied that one element is generated substantially from another; on the other, he posited that all things were originally one through confusion and chaos, as though Friendship had confused them all. This could not have occurred without corruption. Or, if there was no corruption and generation, how did many differentiated things emerge from that one?

Summary of chapter 2

Aristotle does two things in this chapter. First, he presents two other opinions of the ancients concerning the manner of substantial generation; second, he argues specifically against the second opinion and resolves its argument.

As to the first, Aristotle teaches that by then it was a common opinion that generation and alteration were distinct, and that generation occurred through the aggregation of certain things, just as corruption occurred through their disaggregation, while alteration occurred through the transformation of certain qualities. Plato and Democritus and Leucippus held different opinions about the aggregation from which generation occurred. Plato said generation occurred through the aggregation of surfaces or lines; Democritus said it occurred through the aggregation of atoms or indivisible bodies.

As to the second, he presents Democritus’s argument and resolves it.

The argument was this: Natural bodies are composed either of indivisible bodies or of divisible ones. If the first, the point is established. If the second, those divisible things could be divided again. If every possible division were fully carried out, what resulted or remained would either be a divisible body—which is impossible, since we suppose that every division has been carried out—or nothing, in which case a body would be composed of nothing, but nothing comes from nothing; or points, which is impossible, since otherwise a body would be composed of points alone, but a point added to a point does not make anything greater, because it has no quantity; or, finally, qualities, which is also impossible, since otherwise a body would consist of qualities alone, and those qualities, thus resolved, would be separate from body.

Aristotle replies that a body is divisible successively and syncategorematically, and potentially, into other divisible things; it is not possible for every division to be posited in act. Rather, something is divided in act in such a way that it still remains divisible. For example, we say that different things can be generated from the matter of air, but not simultaneously; something always remains in potency to something. Finally, he shows that natural generation does not occur through the aggregation of different things, because generation does not occur through the position and arrangement of parts as in artifacts, such as a ship or house, but because this whole is dissolved and another whole is generated.

Summary of chapter 3

Aristotle does three things in this chapter. First, he inquires whether generation simply speaking exists, that is, whether there is generation by which something comes to be simply and substantially. Second, he presents two difficulties necessary for resolving the question. Third, he resolves the principal question.

As to the first, he presents the reason for doubt: if something comes to be simply, it must come to be from its contrary, namely, from what is simply nonbeing. But this is impossible, because that from which something comes to be exists in what comes to be and is predicated of it. For example, the wood from which a chest is made is in the chest and is said of it. Thus nonbeing would exist in being and be predicated of it.

As to the second, he presents two difficulties on which the resolution of the question depends. The first is why generation never ceases on the part of the material cause. For if in generation something comes to be simply, then in corruption something ceases to be simply, and thus some entity is consumed through corruption. But since the entire entity of the universe is finite, it would eventually be consumed through the corruptions of things.

He replies that the cause is not that matter is infinite, since there is no actual infinite; nor is it that the things generated become smaller and smaller over time, since we do not see such diminution, but things are generated equal to their generators. Rather, the cause is that the corruption of one thing is the generation of another while the same matter remains. Thus finite matter suffices for infinite generations.

The second difficulty is this: if the corruption of one thing is the generation of another, why are some things said to be generated simply, as when a human being comes to be from an embryo, while others are generated in a qualified sense, as when a corpse is generated from a human being? It is said that the human being dies rather than that something is generated.

The reply is that this must be judged from the term of generation. Sometimes a complete and ultimate form is acquired; sometimes an intermediate form is acquired, one that prepares the way for another. When a form of the first kind is acquired, something is said to be generated simply, because generation comes to rest there, as when a human being, horse, or stone comes to be. In the second case something is said to come to be in a qualified sense, because something still remains to be made: when the form of an embryo comes to be, a human being remains to be made; when a corpse comes to be, earth remains to be made, toward which the form of a corpse tends. Likewise, when an accident is generated, generation is in a qualified sense, since substance is being simply. Finally, when something is generated that is not easily perceived, such as air, people commonly say that something is generated in a qualified sense.

As to the third, Aristotle replies that generation simply speaking exists. The reason is that there is a process toward being simply, as toward the form of fire, from the nonbeing of that form, which is nonbeing simply in matter. Therefore there is generation simply, just as there is corruption, since conversely that being simply is changed into nonbeing. What is the common subject of both forms, namely matter: is it nonbeing? He replies that it is a being in potency, not a being in act. It has being as nonbeing only by reason of the privation joined to it, because it lacks form. Thus generation does not occur from nonbeing in every way, but from a being in potency.

Summary of chapter 4

Having explained whether generation simply speaking exists—that is, whether it occurs—in this chapter Aristotle explains what it is, doing two things. First, he teaches what generation and alteration are; second, he assigns their differences.

As to the first, he presupposes that accidents differ from substance because they inhere in it, as whiteness inheres in a human being, and that change can occur concerning both. For we see that sometimes a human being perishes substantially, and sometimes changes accidentally, such as in color or heat.

Given this, he says that alteration is change concerning quality while the same sensible subject remains, as when water that was cold is heated. Substantial generation, however, is change according to the whole substance or subject of a thing; therefore the same sensible subject does not remain, as when fire is generated from wood. Only prime matter remains, and it is not sensible in itself, but only by reason of forms.

As to the second, in presenting the differences between alteration and generation, he presupposes that in generation there is sometimes also a transformation of qualities, as when hot fire comes to be from cold water; sometimes there is no change of qualities, because a similar quality remains in the generated and corrupted things, though not a contrary one. For example, when air comes to be from water, transparency remains in both.

Given this, he assigns two differences between alteration and generation. The first is on the side of the term produced: alteration produces a quality, while generation produces a substantial form. The second is on the side of the subject: the subject of alteration is matter informed, or made actual; the subject of generation is prime matter, informable and a being in potency.

Summary of chapter 5

From this chapter the Philosopher begins to treat what follows or accompanies generation. First he treats increase and nutrition in this chapter, doing three things: first, he presents the difference between increase, alteration, and generation; second, he explains the way increase and diminution occur; third, he explains how nutrition differs from increase.

As to the first, he says that increase differs from alteration and generation in two respects. First, on the side of the terms toward which they proceed: alteration proceeds toward quality, generation toward substance, and increase toward quantity. Second, what is generated or altered does not necessarily change place, as when wood is corrupted and fire comes to be in the same place. Increase, however, since it enlarges quantity, necessarily requires some change in the place in which it extends; diminution will have the opposite character.

As to the second, to explain the manner of increase, he presents and resolves two difficulties, then draws two corollaries or suppositions.

The first difficulty is whether what increases requires the addition of something that has magnitude or lacks it. He replies that the nourishment from which increase occurs must have quantity: first, because it must have matter, which does not exist without form, nor form without quantity; second, because it must add quantity to what was there before, so it must have some quantity.

The second difficulty is this: given that both the body to which increase comes and the nourishment itself must have quantity, which of them is said to increase? He replies that it is not the nourishment that increases, but the body to which the nourishment comes through that increase. The reason is drawn from three conditions required for increase. First, what increases must increase in all its parts. Second, something new must be added to each and every one of its parts. Third, what increases must remain the same in number. Since the nourishment does not remain but is converted into the substance of the living thing, whereas that which is nourished and into whose substance it is converted remains the same, it is plainly this body that increases.

He presupposes as certain two things. One is that similar parts—that is, parts of the same nature in the whole and in its parts, such as all the flesh or all the bone—are increased first through increase. Dissimilar parts, for example the foot, hand, and arm, are said to increase by reason of the similar parts, since they consist of such parts along with others. The other is that increase occurs according to formal parts, insofar as the living thing remains numerically the same throughout its life; this is verified according to formal parts.

As to the third, he concludes that two changes intervene in the increase of a living thing: one substantial, by which the nourishment is corrupted and takes on the form of the living thing; the other accidental, by which a greater quantity is acquired. These changes agree in one respect: both occur through some addition of nourishment and quantity. They differ in two respects. First, increase proceeds from a quantity in potency, whereas nutrition proceeds from that same thing insofar as it is potentially flesh or bone. Second, nutrition lasts throughout life, but increase does not; increase lasts only as long as the power is strong enough to convert more than it loses.

Summary of chapter 6

In this chapter the Philosopher treats the contact required between agent and patient, doing two things. First, he explains why bodies’ contact must be discussed; second, he explains three modes of contact and which of them pertains to the present subject.

As to the first, he shows that contact must be discussed because one must treat action and mutual reaction, as well as the elements and the mixtures made from them. All these require contact, and therefore contact must be treated.

As to the second, he presents three modes of contact. The first follows mere local position and belongs even to bodies considered mathematically. It is defined as follows: “Those things touch whose extremities, that is, surfaces, are together and have determinate magnitude.”

The second is physical contact properly so called, namely, when active and passive things act and are acted upon mutually, and their extremities are together, as air and water when they act mutually upon one another and are joined. It is physical contact because it occurs according to motion, or action and passion.

The third is contact so called in a broad sense, because it is neither mutual nor reciprocal: one thing acts without being acted upon by the other, as intelligences touch the heavens, the heavens touch these lower things, or what saddens touches what is saddened.

Only the second meaning of contact pertains to the present subject.

Finally, he concludes two things. One is that agent and patient have contrary qualities. The second is that being moved is more common than acting, if acting is taken strictly as opposed to being acted upon, which involves corruption. In this sense, acting is moving with a disposition toward corruption, and this does not belong to every kind of moving.

Summary of chapter 7

In this chapter and the next two the Philosopher treats action and reaction. In this chapter he explains the condition of agent and patient both absolutely and when they react upon one another. In the following chapters he presents two opinions of the ancients about the manner of acting and being acted upon. Finally, in chapter 9, he explains from his own opinion four conditions required for acting and being acted upon.

In this seventh chapter, then, he does three things. First, he explains the condition and nature of agent and patient with respect to whether the patient must be like or unlike the agent; second, whether the agent is acted upon in return by the patient; third, to which kind of cause agent and patient belong.

As to the first, he first reports two opinions of the philosophers. The first said that agent and patient must be unlike; for if both are alike, there is no greater reason why this one acts rather than that one. The second said that they must be alike, because things that are disparate and unlike neither act nor are acted upon, as white does not act upon sweet, or upon heavy or light, and so on.

Aristotle rejects both opinions as incomplete. He proves that they cannot be altogether alike, because there would be no reason why one should be agent and the other patient; moreover, otherwise something would act upon itself much more if it acted upon what was wholly like it. That they cannot be altogether unlike is clear because agent and patient cannot be wholly diverse, as heat and quantity, which do not act upon one another. Rather, they must be contraries; but contraries belong to the same genus and thus have some agreement and likeness.

From this he concludes that agent and patient must be partly unlike, namely, at the beginning of the action, since they are then contraries, and partly alike, namely, at the end of the action, since the agent intends to make the patient like itself.

As to the second, namely, whether the agent is acted upon in return by the patient, he presupposes two divisions. The first is that a mover is either first, namely, that from which motion first begins, or ultimate, namely, what is immediate to the mobile thing. The second is that an alterer is either first—that is, something that alters without being altered, such as the heavens, or a physician who heals without being altered by the patient—or ultimate, namely, what per se reaches the patient, as a medicine does in healing.

He therefore replies that when agent and patient have matter in common and of the same kind, the agent can be acted upon in return, as all sublunary things can, because they have common matter, that is, matter mutually transformable. Things that do not have matter in common, such as the heavens with respect to sublunary things, act without being acted upon in return. In a word, the first agent or alterer is not acted upon in return, but the ultimate one is.

As to the third, he teaches that the agent belongs to the genus of efficient cause. First, alteration begins from it, and the efficient cause is that from which motion begins. Second, it does not belong to another genus of cause: not to final or formal cause, because an agent acts in act when the action proceeds from it, but at that point neither the end nor the form yet exists, since if either existed the action would cease. Nor does it belong to the genus of material cause, since matter is acted upon, and thus the patient belongs to it instead.

Summary of chapter 8

He does two things: first, he presents three opinions of the ancient philosophers concerning the manner of acting and being acted upon; second, he refutes them.

As to the first, the first opinion was Empedocles’, who held that bodies acted upon and were acted upon by one another insofar as one has pores or cavities into which another enters and as it were penetrates; thus mixture and mutual action occur.

The second opinion was held by Democritus and Leucippus, who posited the same pores, but filled with atoms or small bodies. Since these were discontinuous, they left some void through which they moved. Generation occurred by their aggregation, and corruption by their disaggregation.

The third opinion was held by Parmenides and Melissus, who posited that there is absolutely one thing in the world, but that it is segregated by certain voids. When the void is removed, all things are one. They likewise required a void for motion, since they said that if a body moved through what was full, bodies would penetrate one another.

As to the second, having compared these opinions with one another and observed that it makes little difference in what they agree and differ, he attacks the two opinions concerning pores and atoms.

He reduces Empedocles’ opinion to the second. They differ only because the second opinion posits those voids as filled with small bodies or atoms, whereas the first posits only openings or pores into which other small bodies enter in order to act. He says, however, that these pores are posited needlessly to explain this mode of action, because the small bodies entering them either touch the solid parts or do not. If they do not, they do not act within those parts either. If they do, they will touch them even without pores, by approaching from outside.

He attacks the second opinion about atoms at greater length. Briefly, it is disproved because according to this opinion action and passion occur through the void, but the atoms themselves do not contain a void; therefore they neither act nor are acted upon by one another. Also, everything that acts is soft or hard, or affected by some similar quality. But an indivisible body is neither soft nor hard, and so on; therefore it neither acts nor is acted upon.

Summary of chapter 9

In this chapter, to establish the true opinion concerning the manner of action and passion, Aristotle does two things. First, he presents the conditions required for acting and being acted upon; second, he shows that Democritus’s opinion about atoms destroys generation, alteration, and increase.

As to the first, the conditions required for action and passion are reduced to four. First, the agent must be in act and the patient in potency, as fire acts insofar as it is actually hot, while wood is acted upon insofar as it is in potency to heat. Second, agent and patient must touch, either immediately, if the agent is ultimate and immediate, or mediately, if it is remote, as the sun touches the earth through the air. Third, agent and patient must not be continuous, that is, alike and of the same kind in acting; for things continuous in quantity can indeed act and be acted upon, as a hot part of a body heats a part that is not hot. Fourth, they must be at the proper proximity, with one within the sphere of the other’s activity.

He excludes a fifth condition posited by some, namely, that the patient is not acted upon according to every part. This is false, because if the patient is divisible and has parts, then if the agent is applied to all of them, it will be acted upon in every part.

As to the second, he shows that Democritus’s opinion, reported in the preceding chapter, eliminates alteration and increase. It eliminates alteration because it is clear that a whole body can be altered by heat or cold without any division of the body; in that opinion, however, atoms are aggregated within pores in generation and disaggregated in corruption, which cannot occur without division. It eliminates increase because in increase every part must increase; but in that opinion there is no part that increases, since everything is indivisible. Rather, atoms would merely be placed beside atoms, which is not intrinsic increase but increase by juxtaposition.

Summary of chapter 10

In this final chapter the Philosopher treats mixture, which is especially relevant to the generation of things made from the elements. The chapter is divided into three parts: first, he shows that mixture exists; second, he explains how mixture occurs; third, he gathers what is mixable and what mixture itself is.

As to the first, he reports the opinion of those who deny mixture, with this argument: things that are mixed either remain as they were, or are corrupted, or one remains while the other is corrupted. If the first or third, there is no mixture, because what remains as before is not mixed, since it is not changed. If the second, there is no mixture from a thing that has been destroyed and no longer exists. Therefore, and so on.

The Philosopher replies that mixture exists and distinguishes it from other changes. Mixture does exist, because the mixables remain in the mixture not in act but virtually and potentially; thus mixture comes to be from them as altered and remaining through their qualities. It is distinguished from the other changes as follows. It differs from simple generation because in simple generation something is corrupted entirely, whereas in mixture something is generated through the concurrence of several things, which persist virtually. It differs from alteration and increase because what is altered and increased remains the same after alteration and increase; what is mixed, however, does not remain the same after mixture but is changed into something else.

As to the second, he first reports two opinions of the ancients concerning mixture. The first said mixture occurs by division into the smallest parts, which are divisible in themselves but appear no longer divisible to the senses, as when wheat flour is mixed with barley flour. The second said mixture occurs by division into minima that are no longer divisible not only to the senses but also in themselves.

The Philosopher refutes both opinions, because on this account mixture would be nothing other than the aggregation in one place of several things, each remaining whole in itself and separate from the others, although the senses could not discern this. It would further follow that a mixture is not homogeneous, that is, that every part of the mixture is not mixed, although we see the opposite: mixture belongs to the whole and its parts. It is also incompatible with a continuum’s being divided into parts that are no longer divisible; otherwise a continuum would be composed of indivisibles, a claim refuted in Physics VI.²

He teaches the true manner of mixture by explaining three conditions for it. First, the mixables must be mutually active and passive; things that do not act on one another are not mixed, as things that do not have matter of the same kind. Second, they must yield readily and be divisible, as soft and moist things mix readily, unlike hard things such as stone and iron. Third, they must not be too large, because such things are altered with difficulty; rather, they must be divided into small parts, so that one can enter another and act upon it, readily altering it. Thus mixture occurs according to these conditions in such a way that the activity of one is proportioned to the resistance of the other, and they come together in one temperament. For if one greatly overcomes the other, there is no mixture but simple corruption and generation, as when a drop of water falls into a jar of wine.

As to the third, he teaches that what is mixable is that which acts and that which is acted upon, so that from both a mixture comes to be according to a temperament. Mixture, however, is “the union of things mixable and altered.”