Philosophy of Nature: II. Change and Motion
Motion
For Aristotle, motion is the technical name for the process of something acquiring a new actuality gradually or by stages. While all change is the acquisition of a new actuality or form by something in potency to it (matter), motion refers to those processes whereby such changes occur progressively, the potency becoming more and more actual by degrees until the new form is fully actual. Aquinas in his Commentary on Aristotle’s Physics thus accepts Aristotle’s definition for motion as “the act of a being in potency insofar as it is in potency,” and explains,
For when water is hot only in potency, it is not yet moved; but when it is already heated, the motion of heating is finished. When, however, it already participates in something of heat although imperfectly, then it is being moved to heat. For that which is becoming hot gradually participates more and more in heat. Therefore, motion is that imperfect act of heat existing in the heatable, not, indeed, insofar as it is only in act, but insofar as already existing in act it has an ordering to further act. … Hence, the Philosopher defines motion most aptly by saying that motion is the entelechy, i.e., the act, of a thing existing in potency insofar as it is such [i.e., in potency].[1]
As a process, motion is the action of a substance, for instance, progressively losing one accidental form or actuality and gaining another. As such, motion is an actuality, but an imperfect one. Hence, the definition includes the qualification “insofar as it is in potency.” Motion is the act of something that does not yet have, but is actively acquiring through the action of an agent, the full actuality of a new accidental determination, a new quality, size or shape, or location. What is undergoing a motion is active, but not with the full actuality it will have when the motion is complete. While the motion is taking place, the new determination is neither fully actual (for then the motion would be over) nor is it fully potential (for then the motion would not have begun).
Since motion entails both act and potency, it involves both perfection and imperfection. It involves perfection insofar as it is an act: the actual process of attaining some new perfection. It involves imperfection insofar as it entails potency. For this reason, motion may be described as an “imperfect act” or the “act of something imperfect.” (In meta. XI, lect. 9, (§ 2305); In phys. III, lect. 3, no. 6 (§ 296)) It is an imperfect act in that it is the act of something on its way to further actualization. Thus water which is in the process of being heated enjoys an actuality of heat greater than what it had before the process started, but not yet so great as what it will have when the process is completed.[2]
As we will see when we look at Aristotle’s and Aquinas’s understanding of causation, especially efficient causes, every motion, like change in general, must have an agent which makes some potential to be actual. The fact that motion is an imperfect act implies, for Aquinas, that for every true motion, there must be a cause sustaining that motion. In change in general, there must be a cause, but the action of that cause ceases once the change is complete. Once the marble is David-shaped and its potency is actualized by the new accidental form (shape), Michelangelo no longer acts as an agent. Michelangelo is the agent cause of the completed statue of David, but while sculpting it, he is the cause of the sculpting motion.[3]
Kinds of Motion
Since motions are gradual, progressive processes of acquiring a new form, only those changes characterized as form gradually coming to be in matter are motions. Recall that among accidents, only location, quantity, and quality are commonly described in this way since these attributes can be had as more or less, to greater or lesser degrees. Again, one does not normally, even for Aristotle and Aquinas, describe becoming a father as ‘receiving the form of fatherhood’ (relation), or beginning to hammer a nail as ‘receiving the form of hammering’ (action), or ‘receiving the form of being clothed’ (habit), The acquisition of the new accidental determination or actuality in these cases is instantaneous; one either is or is not a father or hammering or clothed, and one cannot be more or less a father, hammering, or clothed (though the amount of clothes one has on (or not) may be more or less).
In contrast, Aristotle and Aquinas describe the changes in the first three genera of accidents as motions, processes of coming to have a new actuality of the appropriate sort. Thus, the process of coming to be in a new location is what we now usually mean by motion, i.e., local motion, the traversing of some distance (in a certain amount of time[4]). Likewise, motion also applies to a change in quantity (size, weight, or shape as the limit of a thing’s surface), such that growth and diminution are another species of motion. As a plant or an animal gets taller or shorter or changes shape, it undergoes a motion whereby its height or weight or shape becomes more or less until it attains the final quantity that defines that change. Finally, the term ‘motion’ also commonly applies to a gradual change in quality (reddening or darkening, heating or cooling, becoming moist or dry, etc.), and it receives the name alteration, the gradual replacing of one quality by a contrary one.
One might expect that the generation of non-living substances, e.g., the generation of ashes from the burning of a log or of rust from iron, would be a motion for Aristotle and Aquinas, since the new substances (ashes and rust) come about from a gradual process of the corruption of the log or iron. Indeed, in chapter 1 of Book III of his Physics (200b30, 201a14), Aristotle lists generation and corruption as a kind of motion along with local motion, growth/diminution, and alteration, and Aquinas presumably concurs. But later, in Book V, chapter 2, Aristotle explicitly excludes generation and corruption of substances from being motions since substantial forms do not admit of degrees, i.e., of being more or less (225b10). A substance cannot be more or less the kind of thing it is, and so cannot come to be gradually or by stages. Something cannot be partially or mostly, but not completely, a dog, say, on its way to becoming a fully actual dog. Fido either is a dog, or he is not (at all); either the sperm and egg of his parents will be (but are not) a dog or his corpse was a dog (and is an ex-dog). Aquinas, in his own writings, acknowledges this fact.
[S]ubstantial form is brought into act, not continuously or gradually, but instantly (otherwise there would have to be motion in the genus of substance just as it is in the genus of quality) … Thus, the form of fire is not induced in the air in such a manner as it proceeds gradually from imperfect to perfect, since no substantial form is subject to more and less. Rather, only the matter is modified through a preceding alteration so as to be more or less disposed to form. The form, however, does not begin to be in the matter until the last instant of alteration.[5]
Processes that seem to be the gradual coming to be of substances are not, in fact, one continuous gradual process, but a series of discrete, instantaneous changes in substances. As we will see later, especially in light of the modern understanding of chemical changes, a log or piece of iron, are not each actually a single substance. Rather, non-living substances only seem to exist as individual wholes at the level of molecules. A log, then, is a collection of chemical elements and molecules having lost the cohesion and substantial unity they enjoyed being the matter of the living tree, from which the log came to be, its non-living remnants. So, when a log burns or iron rusts, in actuality these collections of molecules become ash or rust by individual molecules instantaneously undergoing substantial changes: wood (cellulose, lignum, et al.) and oxygen to smoke and ash; or iron, oxygen, and water to rust (ferric oxide) and water. (More on this under Material Cause, below.)
Aristotle and Aquinas, however, thought that the substantial generation of complex animals, including humans, to be more complicated than that of non-living things, but it still was not as a motion, but in the end instantaneous. The process of animal and human generation was not a gradual appearing and actualization of one final substantial form, i.e., soul, as the motion of altering accidental qualities is. There are motions (alterations), but these led to a succession of substantial forms which prepare the offspring’s body to be fit matter for its final substantial form.
It should be known that the generation of men and animals is different than the generation of air or water. For the generation of air is simple, since in the whole generation of air there appear only two substantial forms, one that is displaced and one that is induced, which wholly comes about at the same time in one instant, so that before the introduction of the form of air, the form of water continues to remain there. In the generation of an animal, however, various substantial forms appear: first the semen appears, then blood, and so on until there is the form of a man or an animal.[6]
Aristotle and Aquinas have rather elaborate accounts of embryonic development of higher animals, including humans, and they are sometimes cited in contemporary moral and political debates as they concern the coming to be of new human life. They are, however, in the light of modern biological facts, very much mistaken. Even on their own hylomorphic principles, they involve some major philosophical difficulties. For now, it is enough to note that, on Aristotelian and Thomistic principles, the substantial generation of animals is never one simple motion in their technical sense (though it may involve many such motions as well as many instantaneous substantial changes). We will consider elsewhere the details of the Aristotelian-Thomistic account of the generation of animals and humans, and why modern biology shows how this account does not conform to the truth of things.
Thus, with the generation of substances having been excluded, there remain three kinds of motion for Aristotle and Aquinas:
- change in quantity, size, or dimension (called growth or diminution)
- change in place (called local motion)
- change in quality (called alteration)
Aristotle and Aquinas typically describe the last, alteration, especially in terms of the receiving of form into the (secondary) matter of a substance. Thus, when fire heats water, the water undergoes the motion of being heated whereby it receives from the fire the accidental form (quality) of heat. Even though they say that the water receives the form of heat from fire, they are clear that the accidental form does not move or transfer from the fire to the water.[7] Instead, the fire as agent induces heat in the water, or better, educes from the potency of cold water the new accidental form of heat. The fire and the water, at the end of the motion, come to share the same accidental form, i.e., heat (up to the limit of 100o celsius, at which temperature water becomes steam (‘air’ in Aquinas’s example, above)). The heat of the fire and the heat of the water, though, are specifically the same, the same in kind, but not identical, the same in number.
By motion, the agent does not surrender or hand over its own form to the mobile object. Rather, the form belonging to the agent is the active principle from which a given motion proceeds. By this motion, the potentiality of the mobile object is actualized or perfected. It is in this way that a (perhaps) specifically identical, but always numerically different, form may be educed from the potentiality of the mobile object.[8]
The heat of the water is its own and was caused by the fire, but did not ‘move’ from fire to water, for an accidental form would have to have some subject (substance) in which to be ‘moved’ from one place or substance to another. In general, causes do not transfer accidental forms but, by their actuality, they make the potency of what they act upon to be (or become) actual. Agent causes, by their actuality, reduce the potency of patient substances to actuality, but do not ‘transfer’ that actuality or accidental forms, except in a very metaphorical way. Some agents cause the same accidental form in what they act upon, but some cause their effect through an actuality that is different in kind, as when friction (local motion) or the sun produces heat. We will discuss different kinds of causal agency below when we turn to Efficient Causes.
Two Kinds of Alteration
Aristotle and Aquinas after him, note that, in addition to motions or alterations in this ordinary sense, there is another kind of active process, called immanent activity or operation, which is a motion or alteration in an analogous sense for it shares some of the same characteristics but with pretty significant differences. Motion with respect to quality, which we have considered so far, is technically termed “alteration” properly so called (also called “transient motion (or change or immutation).”) It is the gradual process of change brought about by an external agent actualizing the potency of the affected substance (patient) to acquire a new quality: to become hot or cold, moist or dry, light or dark, etc. There are four characteristics of alteration in the strict sense that are proper to it, and by which Aristotle and Aquinas distinguish immanent activity from this first, primary, sense of alteration. First, alterations come about in passively affected substances, whereby they ‘receive’ a new actuality from the activity of some agent extrinsic to it. Immanent activities, by contrast, remain in the agent and do not pass into a patient. In vital activities – nutrition, sensation, intellection – living things have operations that remain intrinsic to themselves, even though such operations have an object and involve a relation to something extrinsic to themselves.
Second, alterations are incomplete until the new actuality is fully realized. When water is being altered and undergoing a motion while being heated by the fire, the heating ends only when the water reaches the desired temperature (or the maximum of which water is capable (while still being liquid) – 100o C). With immanent activities or operations, they are complete as soon as, and for as long as, something (a sensing or desiring animal or a knowing or willing human) is engaged in them. When a lion, for instance, sees its prey, not only is it in the process of seeing, but that process is also complete: the lion has seen, is aware of the prey; it is not merely becoming aware.
Third, in alterations or motions toward acquiring new qualities, when the new actuality is fully realized, it replaces a contrary actuality or form (which, it will be remembered, was the privation of the new form or actuality). Water is altered by fire when the form of heat replaces the form of cold or tepid. In immanent activities or operation which receive forms (sensation and intellection), by contrast, new forms are received without displacing prior forms, and yet they fulfill and complete the underlying form (sensitive or rational soul) in which the powers for such receptions reside. Alterations in the strict sense means that one quality of a substance, cold, for instance, is destroyed or replaced by another, contrary quality, heat, in this example. But in immanent activities, a form received by the potency (the cognitive faculty or power: sense or intellect) does not replace prior forms but fulfills or perfects the power (it develops into or actualizes its real self). By such operations, the animal or human senses or understands the thing whose form it received.
Finally, the new form brought to actuality in the substance undergoing alteration is received in a ‘material’ manner, as having the quality in a literal sense, (often, but not always) as the agent which caused it (see virtual and eminent causation, below). That is, heat comes to be in the water in the same sense as in the fire. Even when heated by an agent that is not exactly hot (microwaves), the water becomes literally, i.e., materially, hot. However, when Aristotle and Aquinas say that in immanent activities (sensation and intellection), forms are received ‘without matter,’ they mean that the recipient comes to have that form in a non-material way, i.e., not as the form would be in the matter of standard alterations, and not as the form is had materially in (most) agent causes of standard alterations.
We will have much more to say about the immanent activities or operations of sensing and knowing in when we consider them in activities of the souls of living things. We mention the difference between transient alterations or motions, on the one hand, and imminent alterations or operations, on the other, here in order to outline the ways the motions or alterations that non-living substances undergo differ from the activities of living, especially sensing, natural, physical substances, and to emphasize the fact that some processes that seem to be motions or alteration in living things are not. As we have said, we will have much more to consider in the next chapter about cognitive operations as the immanent activities of senses or the intellect.
[1] Bk III, L 2, n. 285.
[2] Dodds, op. cit., p. 93.
[3] Aquinas employs motion in this technical sense (as applying to gradual changes in quantity, quality, as well as location) in his First Way of proving the Existence of God in the Summa Theologiae (ST Ia, q. 2, a, 3), a point often overlooked by his interpreters and critics. We will look at this proof more closely in Chapter 6.
[4] Time, for Aristotle and Aquinas, being the measure of motion.
[5] On the Power of God, q. 3, a. 9, ad 9.
[6] On the Power of God, q. 3, a. 9, ad 9. Cf. Summa Contra Gentiles, II, Ch. 89, 11; Summa Theologiae, Ia, q. 119, a. 2.
[7] Aristotle thoroughly debunks theories of Empedocles, Leucippus and Democritus to this effect in On Generation and Corruption, Bk. I, Ch. 8 (324b26-326b24). See Joseph M. Magee, Unmixing the Intellect: Aristotle on Cognitive Powers and Bodily Organs (Greenwood 2002), pp. 102-3.
[8] Dodds, op. cit., p. 97.
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