JMC : The Physical System of St. Thomas / by G.M. Cornoldi, S.J.


IN general all change whatsoever is motion. One Being only is in rest. That Being is God, Who is immutable. All created things may be said to be more or less in a state of change and motion, especially corporeal things, whose nature and operations have to be considered by the physical philosopher. Let us begin therefore by giving Aristotle's definition of motion: "Motion is the act of Being that is in potentia as such."

This at first sight may seem somewhat obscure, especially to anyone who, having accustomed himself to be satisfied with descriptive and superficial definitions, cares little or nothing for those that penetrate and explain the essence of things. But let us see what St. Thomas says about it in his commentary on Aristotle's Physics. "We must know," he says, "that some have defined motion as the non-instantaneous coming forth of the actual from the potential: but this definition is erroneous, because they put into it that which presupposes motion. Coming forth is a species of motion: and the word instantaneous regards time, instantaneous being that which happens in an indivisible point of time, and time being defined by motion. Hence it is impossible to define motion by previous notions better known (per priora et notiora), except as the Philosopher defines it. For, as we have said, everything is divided into the potential and the actual. But potentia and actus, being among the first differences of being, are naturally prior to motion: and the Philosopher uses them in defining motion. We must consider therefore that some things are actual only, and others potential only, and others midway between the potential and the actual. Now that which is in potentia is not yet moved. That which already is in aetu perfecto is not moved, but has been moved. That which is being moved is midway between the potential and the actual, being partly in potentia and partly in actu: and this is evident in alteration. For water, when hot in potentia only, is not yet moved or changed. When already hot, the movement of heating is ended. But if it participate of heat imperfectly, it is then moving towards being hot; for that which is becoming hot participates of heat gradually by little and little. Therefore that imperfect act of heat in the thing that is being heated is motion, not as actual only, but inasmuch as, being an act, it has a disposition for a further act, because, if that disposition were taken away, the act, however imperfect, would terminate the motion, instead of being motion, as when anything is incompletely heated. But the disposition for a further act belongs to what is potentially that further act. In like manner, if the imperfect act be considered merely in order to the further act, according to which it means a potentia, it has not the essence of motion, but of a principle of motion, for heating may begin from what is tepid, as well as from what is cold. Thus therefore the imperfect act has the essence of motion, as a potentia, in comparison with an ulterior act, and as an act, in comparison with something more imperfect. Wherefore motion is neither the potentia of that which is potentially, nor the act of that which is actually. It is the act of what is in potentia, the word act referring to an anterior potentia, and the word potentia referring to an ulterior act. Most suitably therefore did Aristotle define motion to be the entelecheia, i.e. the act of Being existing in potentia, as such: Motus est actus existentis in potentia secundum quod hujusmodi.{1}

Evidently this definition of motion is general; defining the genus, not the species. So considered, motion requires two terms, and, like a tide, ebbs and flows between them. One term is the terminus a quo, and is the principle of motion. The other is the terminus ad quem, and is the end of motion, viz., rest. The thing in motion cannot have reached the terminus ad quem, or it would be at rest; nor can it be in the terminus a quo, or it would not yet be in motion. It must be between the one and the other, leaving the former and tending towards the latter. Every change is precisely the transit from the one term to the other. In the former case the thing may change. In the latter it has changed. Between them it is changing: and precisely between these two every change is motion. Therefore God, Who is the most perfect and immutable Act, has no perfection potentially, cannot pass from not possessing to possessing, so as to rest in a terminus ad quem. Hence in God all motion whatsoever is impossible. But in all creatures there is, firstly, potentia, secondly, an imperfect act, lastly, a complete act: and therefore that between the two terms a quo and ad quem, has the true essence of motion. Hence in the created intellect, in the will, in the sensitive and vegetative faculties, and in every alteration, there is the generic essence of motion: but in each of these the specific essence evidently varies. And since all creatures, by the necessity of their contingent being, are in potentia -- therefore are imperfect and perfectible -- so in them there is essentially the principle of motion, which, as was said elsewhere, is called nature.

Hence, as St. Thomas remarks with Aristotle, he who has not a true knowledge of what motion is can never have a true notion of nature, and therefore cannot reason rightly about natural things. Natura, he says, est principium motus. . . . Et sic patet quod, ignorato motu, ignoratur natura, quum in ejus definitione ponatur. Quum ergo nos intendamus tradere scientiam de natura, necesse est notificare motum.{2} And here we cannot help noticing, by the way, the wisdom with which in that philosophy the First Cause, the First Mover, Almighty God, was called the Immoveable Being, and all created beings were called moveable. The Essence of God is incompatible with motion. The essence of created beings is, of its own necessity, ordered for motion. Hence, the word nature is, in its proper sense, applicable to created essences, but to the Divine Essence in an analogical sense only.

With regard to the species of motion we may remark firstly that motion, if it means the quality, is called alteration; but if it means the quantity, it is called increase or decrease. Thus through a change of quality a liquid is altered, or an animal through illness, while a plant or animal has a movement of increase if growing, or contrariwise of decrease. Local movement, i.e. the passing of a corporeal substance from one place to another, is called translocation, or change of place.

Here it must be remarked that in physics we have to consider the motion of material beings only. In them the motion, whatever it may be, is generically different from what it is in immaterial beings, as the angels are, and human souls when separated from the bodies which they had informed. In such immaterial beings motion is a non-material change.

And now let us go a little further in considering the said motion of physical things. The first of all motions in order of time is local motion, because the others presuppose it, as the Angelic Doctor, in accordance with Aristotle, tells us in these words:

"He (Aristotle) begins by laying down what he means to show: and he says that, of the three species of motion -- one according to quantity, which is called increase and decrease, another according to quality, which is called alteration, and another according to place, which is called change of place -- the last-named must be the first of all. And secondly, he proves this by the fact of its being impossible that increase can be the first of motions, because it cannot take place without a previous alteration. For that by which anything is increased is in a way similar and in another way dissimilar. It is evidently dissimilar: for that by which anything is increased is nourishment, which at first is different from what it nourishes; but when it does nourish, it must be similar. Now it cannot pass from dissimilarity to similarity without alteration. Increase, therefore, must be preceded by alteration, through which the nourishment passes from one disposition into another. Thirdly, he shows that every alteration is preceded by local motion. When anything is altered, there must be something that alters it, making (for instance) actually hot that which before was potentially so. But if that which causes the alteration were always equally near to the altered thing, it would not cause the heat now rather than sooner. Evidently therefore the mover of the alteration is not always at the same distance from that which is altered, but sometimes nearer and sometimes further. This cannot happen without change of place."{3}

A little further on the holy Doctor speaks of the generation of substances. Without quoting the passage, it will suffice to say that generation cannot happen without the alteration and concurrence of the generators where it should take place. Now all this supposes local motion: and therefore local motion precedes every other motion of corporeal substances.

And now, to examine from a more philosophic and important point of view the doctrine explained, let us consider how some created causes are immaterial and others material. An immaterial cause is a cause that does not in its own being and operation depend on matter; so that matter does not co-operate in the operation as a principle, though it may do so as a term.

Every angel (to say nothing of God) is a cause that can operate on corporeal and material beings; but matter does not co-operate at all therein as a principle of the operation. Hence an angel may in substance and by operation be where a corporeal substance is, and operate on it, and operate at the same time at which the corporeal substance operates: but it cannot constitute therewith one principle of operation. This is philosophically expressed by the saying, that an angel can be the forma assistens of a body, but not its forma informans. A material cause is that which in its being and operation so depends on matter, that matter is a co-principle of operation; as for instance, the principle of activity in any elementary atom, or the vital principle of plants, or the sensitive principle of brutes. These principles cannot of themselves either be or operate : and therefore, to sustain them, they require a subject with which they can operate as a co-principle. Hence the adequate subject of the operations of things inanimate, of plants, and of brutes, is not the principle of activity alone, nor the substantial form only, but, more correctly, the form and the matter together. Therefore these principles and their relative powers are called organic. Now man is the link that unites in himself this twofold causality, material and immaterial; for his spiritual soul is the substantial form of matter, as the anima of the brute is. As being spiritual, it has powers or faculties and operations that are proper to itself and inorganic. Such are the faculties of understanding and of willing. As the substantial form of matter, it has powers or faculties and operations that depend on matter, as those of a brute do. Hence the essential difference between the vegetative or the sensitive powers and those that are intellective lies in this: that the latter are inorganic, and the former organic.

If then the being and operating of an immaterial cause is without matter, it can not when operating, admit in itself local motion, though able to effect it at the term of action whereas, contrariwise, a material cause always operates with local motion. Hence every operation of organic things, of plants, or of brutes, is done with such local motion.

But then in man we must distinguish the operations. Those that proceed from spiritual or inorganic faculties are not done with local motion: but those that proceed from organic faculties, such as the acts of vegetative life, of animal appetites and of imagination, &c., are done with local motion. And since, as philosophy shows, the acts of the spiritual or inorganic faculties presuppose or draw with them those of the material faculties, therefore in man every act of the former is succeeded by a true local motion through the operating of the latter: and evidently this motion, wherever it may take place, will vary according to the variation of the powers and acts by which it is produced. Thus, for instance, it will be different in the imaginative power and in the imagination from what it is in the animal appetitiva and its tendencies; different again in the faculty of sight and the act of seeing; different in the faculty of hearing and the act of hearing; different in the vegetativa and the act of assimilating; and so on, in every operation derived from material power.

It must therefore be maintained, firstly, that a material or corporeal substance operates with local motion: secondly, that such motion will vary according to the operation. Thirdly, therefore, the motion will be immanent where the operation is immanent, i.e. one that has its proper term within the same individual, as it does in living things. Fourthly, it will be transient (transiens){4} where the operation is so, as it is in living things and in things without life, when the substance, that is the cause, operates outside itself. Fifthly, every passion or modification in a material substance that receives the action of another will be done with local motion, which local motion will vary according to the passion or modification.

Anyone who will think a little about these things will easily see how far from the truth are those who, not caring to penetrate to the diversity of essence and of operations in immaterial and material causes, confuse the operations of the latter with local motion, because they always find it accompanied by local motion. They conclude therefore that "in nature there is only local motion," on the principle that hoc est cum illo, ergo est hoc. This is giving all to the senses and nothing to the intellect. By rule of reasoning our conclusion can only be this: If in all the operations of corporeal substances we find, as in fact we do, a true local motion, various according to their variations, we have no right to infer therefrom that in nature there is nothing but matter and motion -- thus excluding every principle of physical activity that causes motion -- but only that the principles of activity, in corporeal, substances and in material powers, operate with motion on matter, and variously according to their various modes of operation.

If the so-called Positivists of modern times, who acknowledge no reality beyond that which is perceived by the senses, had known St. Thomas's sound and true principles of physics, they would not have identified the actions and passions of the sensitive and intelligent human soul with local motion. And those modern physicians, called Psychiatrians, who follow the foolish doctrines of the Positivists, would not have confused that which precedes madness, as a condition, with the disordered reason of a rational soul, and would not, by reducing all to mere motion, deserve to be thought mad themselves as well as mad-makers. But the folly of modern science consists in admitting sensible facts only and condemning as an abuse the use of reason about those facts. This is destructive.

{1} St. Thomas, III. Physic., Lect. 2.

{2} Ibid. In III. Phys. Lect. 1.

{3} Ibid. In VIII. Phys. Lect. 14.

{4} I.e. in the sense of going beyond itself. How can one translate transiens into the anti-philosophic language of Protestant England, where "immaterial" is understood to mean unimportant, "formal" to mean absurdly ceremonious, and the two are marvellously jumbled in "a matter of form"? "Transient" is supposed to mean something not permanent, and "transitive" is applied to nothing but verbs, will any good Christian oblige me with a valid translation of the word? -- Trans.

<< ======= >>