Cursus Philosophicus Thomisticus, Secunda Pars Philosophiae Naturalis

Generation taken generally can be understood as the common notion that abstracts from substantial and accidental generation. For even an accident can sometimes be generated: namely, if it receives being through a change from nonbeing, as in a change, and not only through a change from a contrary, as in motion. In this broadest sense, generation is defined as “a passage or change from nonbeing to being in a subject,” whether the being is substantial or accidental, qualified or absolute.

Substantial generation, taken in general, can mean generation as commonly found in living and nonliving things. It is defined as “the acquisition or change of a substantial form from its privation in matter.” This definition is drawn from various passages in Aristotle where he explained it. For in Physics VIII, text 62,¹ he said that generation is a change from nonbeing to being. And in On Generation and Corruption I, ch. 4,² that it is “a change of the whole into the whole, with no sensible subject remaining,” that is, into matter stripped of sensible accidents. Finally, in ch. 5³ of the same book, he says “substantial generation is a change from substance in potency to substance in act.”

Generation of living things, finally, has a special definition because it proceeds in a special way. It is usually defined in two ways. First, in general, as “the procession of a living thing from something,” which includes both univocal and equivocal generation of living things, whether from another living thing or from something nonliving, as when an animal is generated by heaven. Properly, however, generation of living things is defined when it also occurs through a vital generative action. In this sense it is “the procession of a living thing from a living thing, from a principle of life conjoined to it, into a likeness of nature, at least in the lowest species.” This includes every vital generation and substantial generation of a living thing, even in God, as we shall explain later.

One must nevertheless observe that substantial generation extends more broadly than generation absolutely speaking. Substantial generation is so called in contrast with accidental generation, whether what is generated is a part or a whole, the form that ultimately completes and terminates the entire generation or a nonultimate form that tends toward what is perfect and ultimate. Generation absolutely speaking, however, although it is never said of the generation of an accident, which is always qualified, is not said of every substantial generation either, but only of that by which being is had absolutely and ultimately completed. Hence the generation of a part is not called generation absolutely speaking, but rather a derivative generation by addition (adgeneration); and neither is the generation of an embryo’s form, which is imperfect and, as it were, on its way and tending toward the form of a human being or a horse, called generation absolutely speaking, because generation is not completed in it but tends further. On this, see St. Thomas, On Generation and Corruption I, lectures 8 and 9.⁴

Concerning the definition of substantial generation taken in general, only this difficulty arises: Is it essential to generation that it be a change, or does this belong to it accidentally, so that created generation could be preserved without change? As for uncreated generation, that is certain, since in it there is no passage from potency to act, on which the character of change is founded; rather, there is only a communication of pure act, in which the term is assimilated to the general principle that generates.

Some therefore think that generation is not essentially a change, but that it can be one and that change belongs to it accidentally. Their basis is that it is not essential to generation that the privation of the form to be generated precede it in duration. For, speaking of the connatural order, generation does not occur unless an alteration first takes place, as it were disposing the matter and beginning the generation; and thus it presupposes that the privation of the form to be generated precedes in duration. But the privation need not precede in duration if, at the very instant of generation, God were to produce matter that had not existed throughout all the preceding time, and at that instant introduce the form—as when something is generated from the Eucharistic species. For then, at the instant of generation, matter that remains in the generated thing is supplied by God. Such matter could be supplied even if nothing else had previously existed to take the place of matter and thus to take the place of a thing deprived of form. For example, if matter were newly created by God and an agent that draws out a form were applied to it, that generation could not be a change, because it would not proceed from privation to form, which is essential to the character of change as a passage from nonbeing to being. In that case there is no privation throughout all the preceding time, since the matter did not exist, nor at the very instant of generation, since then privation and form would exist simultaneously.

Nor is it enough to say that privation precedes in an instant of nature, even though it does not precede in an instant of time. The objection is that an instant, or priority, of nature does not designate what is prior in existence and in reality; it signifies only the formal aspect according to which something is said to belong or not belong to something, not whether it in fact belongs or does not belong to it. Thus, for privation to precede in an instant of nature means only that form does not belong to matter according to its essence, or that matter lacks that form of itself—not that it actually lacks it in reality. For in that instant it cannot lack the form, since it has it. Therefore this does not preserve the fact that there is a real change, because in reality it does not pass from privation to form: in reality there is no such privation. Moreover, change requires that a subject be otherwise now than it was before, as St. Thomas says, Summa Theologiae I, q. 45, a. 2, ad 2. But in that case matter is not otherwise now than it was before, since it did not previously exist in the order of things. Therefore there would be no true change.

Nevertheless, one must say: Every created generation that has a material cause from which it takes place is essentially and intrinsically a change.

This is clear first because the Philosopher continually defines generation through change, and thus holds that change belongs to generation’s essence and definition. This is evident in this book, On Generation and Corruption, text 23,⁵ where he defines generation as “a change of the whole, with no sensible subject remaining”; and in text 25,⁶ where generation is a change “from substance in potency to substance in act.” In Physics VIII, text 62,⁷ generation is “a change from nonbeing to being”; and in Physics V, text 7,⁸ it is “a change from non-subject to subject.” Aristotle continually assigns change as generation’s generic character, which means it is essential to generation. Hence St. Thomas explicitly says, On the Power of God, q. 2, a. 1, ad 15,⁹ “generation is a species of change insofar as nature is received through generation in some matter, which is the subject of change.” He therefore presupposes that change is the generic predicate of generation and consequently essential to it. The same follows from those passages where he teaches that generation and corruption are two changes, although they are not two motions, as in On Truth, q. 28, aa. 1 and 6,¹⁰ and Summa Theologiae I–II, q. 113, a. 6, as we shall weigh in the next article. Finally, the Philosopher’s definition of motion in Physics III¹¹ also applies to instantaneous change and generation, as we proved from Thomas and Aristotle in Physics III, q. 14, a. 1.¹²

The reason is that generation essentially occurs between two terms, a starting term and a terminating term. It draws out or introduces a form, which it regards as its terminating term, since it is a tendency toward that term and is specified by it. But if it essentially has a terminating term, it also essentially regards a starting term, because it is impossible to understand a new terminating term coming to be unless from some starting term, whether negative, privative, or positive. Every passage or tendency from a starting term to a terminating term in a subject is essentially a change, since one thing is left behind and another acquired. For neither are both acquired nor both left behind; rather, one is received and the other relinquished, and this takes place in some subject. Therefore generation is essentially a change.

This is confirmed because, through the introduction of a form at the instant of generation, even if privation did not precede in time, matter is formally reduced from potency to act, and the form is truly and properly drawn out from matter’s potentiality. Therefore matter is truly and properly changed. The inference is clear: the potentiality and indifference of matter are found there, reduced to act by the form, and the form itself is drawn out from that potentiality of matter into act. Therefore matter is found to have been changed from potency to act and informed by the form. This informing, in the order of formal causality, removes matter’s potentiality and excludes privation from the resulting actuality. Therefore an actual change of state is given; hence generation, which brings all this about, is a change in coming-to-be, just as the form itself brings about a formal change in the state of actuality.

To the opposing basis we reply that, in the case of generation from the Eucharistic species, we already said in Physics II, q. 2, art. 3, at the end,¹³ that the natural order of generation is not altered there, because quantity, preserved without a subject, takes the place of substance in every respect. Thus it receives the preceding dispositions by which matter is determined for generation, and consequently takes the place of matter deprived of form. For this reason privation precedes in that generation—not in matter, but in what substitutes for matter, namely quantity insofar as it takes matter’s place. At the instant of generation, matter is supplied by God as a constituent of the generated thing, with the same determination toward form that had previously arisen in the quantity through the force of those dispositions, just as if matter had preceded throughout all the prior time. For matter is produced as continuous with what had previously taken the place of matter. Thus that generation occurs from privation, which at that instant is expelled from matter in the state of actuality; throughout the preceding time it had preceded not formally in matter itself, but virtually, that is, in what occupied matter’s place. But in the case where nothing had previously existed, either as matter or as something taking its place and receiving dispositions for generation, we reply that then it suffices for the change found in that case that privation precede by priority of nature, though not by time.

When one presses the point that if privation did not precede in duration there would be no real change from that privation to being, we reply first that if change is removed in that case, generation too is removed; and whatever suffices for generation to remain suffices for change to remain. I add that privation need not precede in duration; it is enough that it be excluded and expelled in reality at that instant by the form introduced by an external agent or drawn out from matter, so that there is change at that instant. Change does not require privation to exist in reality, but requires the real exclusion of privation. Privation is excluded, even if it did not precede in duration, simply because the potentiality of matter is changed by an external agent that draws out a form. Thus the absence of a form is excluded not through its own emanation, but through the action—and therefore the change—of an external agent. Although priority of nature does not signify the existence or duration of a thing, but only that by nature something belongs or does not belong, this suffices for the subject to be said to change when the action of an external agent excludes privation: not a privation that formally inhered, but one that would have inhered if the agent’s action had not removed it by drawing out or uniting the form.

As for the final objection—that change means being otherwise now than before—we reply that, in instantaneous changes, if some motion precedes them as a required disposition, as commonly happens according to the order of nature, then the subject must be otherwise now than before by temporal priority, since those dispositions and motions, and likewise the privation of the form to be generated, precede in time. But if an instantaneous change is considered precisely in itself, with what naturally precedes it set aside, change means being otherwise now than before by priority of nature, provided that privation is prevented from occurring in the subject by the action of an external agent introducing the form. For this is what must determine whether the change occurs without a preexisting privation. Thus at that real instant privation is present not as affecting the subject, but as being expelled from it through an external action. For that priority of nature is not one of existence, but of expulsion and consequently of change at that instant in which the form is introduced.

If you say that for rest it is not enough for something to be disposed in the same way now and before according to priority of nature—otherwise something could be at rest in only one instant—but that it must be disposed as before by priority of time; therefore the same must be required for change, which is opposed to rest, we reply: privative rest requires that something be disposed now as it was before by priority of time, but negative rest does not. The same is required of motion, which consists of two changes, namely generation and corruption, but not of an instantaneous change considered precisely in itself, as when it is not joined to a preceding motion or to the corruption of an opposite form. Motion properly opposed to rest is that which includes two changes, corruption and generation, or is successive motion, in which the subject is otherwise now than before by priority of time. This is not required of instantaneous change, just as it is not required of negative rest.

On the generation of living things

Two phrases in the definition of generation of living things need to be explained. One is how it is “a procession from a principle of life conjoined to it”; the other is “into a likeness of nature.”

To explain the first phrase, one must observe that for the generation of a nonliving thing it is not required that the generated thing or the matter from which it is generated be conjoined to the generator and within it. It is enough for the matter to be placed beside the generator and applied to it externally, so that something is generated from it, as fire generates fire from wood placed beside and applied to it, without being united within it. Living things, however, form within themselves what they generate and provide from their own substance the matter from which it is generated. Indeed, the more perfect the nature of a living thing, the more closely the generated thing is joined to its generating principle. Thus, in intellectual nature, what proceeds from the intellect remains within the intellect not only while it is produced and formed, but for as long as it exists; it is not emitted into another power or subject.

From this St. Thomas distinguishes different degrees of emanation in living things, according to whether the generated thing is more or less closely joined to its generating principle, in Summa contra Gentiles IV, ch. 11, and Sentences III, dist. 8, q. 1, a. 1.¹⁴ Thus we see in plants and animals that seed proceeds and falls away from them; in it are the formative power of the generated thing and the matter from which it is formed. Hence at first the offspring is joined to its generator, as fruit to a tree and the fetus to the womb of an animal, but it is later separated from it at birth. It is therefore said to proceed from a conjoined principle of life both because the matter from which it is formed and the formative power of the seed are produced from the generator, and because it is first joined to the generator and formed within it before birth. In sensitive life, cognition is produced by the very sensory powers and is more intimate to its principle because it occurs through an immanent action. Yet it is not wholly intimate, both because it requires an external principle—namely, an impressed species—and because it produces some term, namely an image or species, in other powers: from the external sense to the common sense, from the common sense to imagination, and from imagination to memory, as will be said in On the Soul III.¹⁵ In intellectual life, the emanation of cognition is still more intimate to its principle. Although in us it requires impressed species that come from outside, the expressed species or word formed through them does not pass to another power, but remains in the intellect itself. Intellectual life is higher still in angels, who do not receive species from outside, since they lack senses; rather, species are born within their intellect or infused there by God. The expressed species also remains within their intellect, but because it is an accident and not a substance, it does not attain perfect generation. Therefore the most perfect generation in every respect is in God, in whom what proceeds is a substance and is so conjoined to its principle as to be one and consubstantial with it, according to the words of John 10:¹⁶ “I and the Father are one.” This entire discussion is St. Thomas’s.

The second phrase is “into a likeness of nature.” It designates what the thing proceeding in generation must be like, and the mode in which it must proceed. For generation of a living thing requires that what proceeds be not only something generated, but also a son. Hairs, nails, worms, and similar things are generated from a living thing: they proceed actively from it, with the living thing supplying the matter from which they are formed, yet they do not proceed as sons, because they are not in the likeness of nature. Second, generation itself must be essentially assimilative—not only concomitantly or in the first mode of per se predication, but also in the fourth mode of per se predication—so that “the generator assimilates” is per se in that mode, just as “the hot thing heats” and “the physician cures” are. For generation is an action that propagates nature; therefore it must produce in the image and likeness of the generator, as though expressing it. This is to be assimilative in the fourth mode, and not only in the first mode of per se predication.

For this reason one understands how the procession of the Word in divine things is generation, whereas the procession of the Holy Spirit is not. The procession of the Holy Spirit is indeed the procession of a thing like its source by identity and concomitance, indeed essentially in the first mode of per se predication, because it is a procession within God and, in the general account, whatever is within God is God. Formally, however, and in the fourth mode, it proceeds only as a divine impulse; because it is divine, what belongs to it is not so much likeness as identity, essentially in the first mode of per se predication, with that from which it proceeds. Thus its procession is not assimilative. The Word, by contrast, according to the very formal character by which it proceeds, proceeds within the intellect as an image and as like its generating principle in the fourth mode of per se predication, not merely by reason of identity because it is within God. Therefore it is most properly generation and an assimilative procession in substance and nature, namely in the divine act of being understood and of understanding. It is like this: if a man could produce another man both from a lump of earth through the art of his hands and through seminal propagation, the one proceeding by propagation would be a son, while the one produced artificially would not be a son, although both would essentially be true human beings. Thus generation requires that something proceed in likeness both on the part of the generated thing itself and on the part of its assimilative mode of proceeding. This is required by the force of generation; for accidentally, because of some impediment, a thing may degenerate from perfect likeness, as in a monster. It would then be a true but defective generation, properly speaking a degeneration.