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


THAT bodies move towards other bodies, to unite with them, is a fact shown continually by experience. But what is the principle of such movement? It certainly is often extrinsic; and when it is so, its motion is mechanical or forced. But often we find it to be intrinsic, and then the motion is physical or natural, i.e. by true attraction. Now what is this mysterious attraction? How does one body attract another from a distance? Where are the means of (so to speak) pulling it? Are these attracting forces invented for the purpose of hiding our own ignorance?

For the sake of clearness, let us begin with a comparison, and suppose that a cow in a field is attracted by some better grass. The grass, by means of light that makes it visible and the exhalation that carries the scent, is made an object for the cow; and the cow, moved by its presence, goes to it. In this fact we have to distinguish firstly the principle of attraction, which is in the grass: secondly the means by which the attraction is communicated, i.e. light and air: thirdly the manner of attracting, which is by acting on the sensible appetite of the cow through the senses and the imagination: fourthly the principle of motion, by which the cow is moved and goes; which is the cow's nature determined actually to go, instead of being merely in potentia to do so. But we must remember, that inasmuch as the substantial form or anima of the cow is the principle of its every operation, the said nature is its principle of motion in virtue of the substantial form. We must remember also that, in order to be such, it must be endowed with some qualities. For the cow would not go after the grass without having first received an impression from it: nor would the impression suffice without the actual going; nor would the cow actually go, if she had not an antecedent disposition to choose the better grass.

This much is clear in living and sensible creatures: but we may speak analagously of inanimate things also. "We find," says St. Thomas, "a certain operation that in one way is common to the animate and the inanimate things, but in another is proper to the animate, such as motion and generation. For spiritual things absolutely have such a nature that they can move but cannot be moved. Bodies indeed move; but, though one of them can move another none can move itself; for, as Aristotle proves in VIII. Phys., those things that move themselves have two parts, of which one is the mover and the other is the moved. But in things purely corporeal this cannot be; for their forms cannot be movers, though they can be a principle of motion by which a thing is moved (UT quo aliquid movetur), as, in the motion of the earth, gravity is the principle by which it is moved (quo movetur,), but is not the mover.{1}

To understand this doctrine then we must remember that, if a substance is to move itself, it must have in it two different parts, one the mover and the other the moved. And this is evident, for in that which moves itself there must be the principle and the term of the motion; and if its parts were quite alike, there would be no sufficient reason why the one should be the mover instead of the moved. Hence living things only, as being composed of various parts differently inclined, can move themselves: and this motion within themselves is called an immanent action. It is not so with inanimate things. These, not having in their parts any diversity of organism, have no immanent action, but only the action that is called in Latin transient: and herein living bodies differ from those without life. Thus we see that, whenever an inanimate body goes to another, it cannot do so by moving itself as above mentioned, but only by transporting its whole self. The attraction then of inanimate things is in this way: First of all, the body attracted will be disposed for going to one body rather than to another. Secondly, the attracting body must act on the attracted through a medium, and, making itself, so to speak, present to it, become its object. Thirdly, the attracted body must receive an impression (in scholastic language passio) sent by virtue of the said medium. Fourthly, having received this, the attracted body must tend actually towards the attractor. And here a question may arise about the attracted body going whole and entire without one part moving the other. To make it clear, we had better begin with an example.

The human soul, being the substantial form of man, does not by one act move the whole body, but, by informing one part, moves another; so that the soul, which informs the whole human body, being the form of a part, is the mover of the other parts. And thus it seems that the motive power originates from the brain and the heart; so that the soul, which informs both, gives by their means movement to the other moveable parts. We may consider the moving part then as a body that gives motion to the contiguous part by transporting its whole self, and thus pressing the part immediately moved by it; for, if we suppose that of the same part one side is the mover and the other the moved, we only remove the question a little, or admit an infinite process repugnant to reason. Here we have the example therefore of a body that in its motion transports its whole self by virtue of the soul, its true substantial form; and therefore it is easy to conceive how a body, even though inorganic, can in virtue of its own substantial form be transported from one place to another.

The Angelic Doctor, speaking of the order and variety of the corporeal universe, finds therein, not a fortuitous collection of many substances, but true dispositions of each for tending mutually to the wondrous formation of the sensible universe.

"All things," he says," seek a bonum, whether they have knowledge or have not. To make this clear, we must know that some, ancient philosophers supposed the effects of nature to arise from necessity of preceding causes, and not because the natural causes had a proper disposition for producing such effects. This the Philosopher condemns in the second book of Physics, where he shows that, if the relation and mutual utility of things were not in some way intended, they would happen by chance, and therefore would not happen in the greater part, but in the less, like all other things that proceed from chance. We must therefore say that all natural things are ordered and disposed for their proper effects. But a thing may be ordered and directed to another as an end in two ways, viz., either by itself, as when a man directs himself to the place whither he means to go, or by something else, as an arrow is directed to a given place by an archer. Now a thing cannot be directed to an end, unless the director knows the end. For that which directs must have knowledge of that to which it directs; but things that know not the end may be directed to a given end. And this happens in two ways. For sometimes that which is directed to an end is only impelled, without receiving from its director any form to adapt it for this or that direction or inclination and such inclination is forced, as an arrow aimed at a mark by an archer. But sometimes the directed or inclined thing has from its director or mover some form by which it is adapted for that inclination; and therefore such inclination will be natural, as from a natural principle. Thus He Who gave gravity to stones inclined them to fall naturally downward; and therefore it is said [VIII. Physics ] that the Maker of heavy and light things is also their Mover. In this way all natural things have an inclination to others that are adapted for them, having in themselves a certain principle of inclination, by reason of which their inclination is natural, so that these in a manner go of themselves, and are not merely led to their proper end (ita ut quodammodo ipsa vadant, et non solum ducantur in fines debitos.) But in forced movement things are only led or pushed, not cooperating themselves at all with the mover but in natural movement things go to their end, inasmuch as they cooperate with the incliner and director by a principle given to them."{2} In accordance with this doctrine Dante wrote

Ond' ella (Beatrice) appresso d'un pio sospiro
  Gli occhi drizzò ver me con quel sembiante
  Che madre fa sopra figlinol deliro.
E cominciò Le cose tutte quante
  Hann' ordine tra loro ; e questo é forma
  Che l'universo a Dio fa simigliante.
Qui veggion l'alte creature l'orma
  Dell' eterno valore, ii quale è fine
  Al quale è fatta la toccata norma.
Nell' ordine ch' 10 dico sono accline
  Tutte nature, per diverse sorti
  Più al principio loro e men vicine;
Onde si muovono a diversi porti
  Per lo gran mar deli' essere, e ciascuna
  Con istinto a lei dato che la porti.

Parad., I.

What has been said shows clearly the meaning of the triple appetitus, so often spoken of by St. Thomas. For, setting aside the forced motion of things, and considering their natural motion only, we find in the first place that man petit aliquem terminum by freely moving himself, because there is in him -- that is, in his intellectual knowledge -- the known bonum that inclines him. Secondly, we find that a brute petit aliquem terminum, not freely, and learns by sensitive knowledge the bonum that inclines him. Thirdly, we find that an inanimate body, whatever it may be, petit aliquem terminum, not by moving itself -- because in it there is not a part that moves and a part that is moved -- but by an inner principle of motion transporting itself to the term. From this we infer that, as principles of motion, there is a rational appetite in man, a sensitive appetite in brutes, and a natural appetite in substances without life.

"As the sensitive appetite," says St. Thomas, "is distinguished from the natural by its more perfect manner of seeking, so is the rational appetite distinguishable from the sensitive for the nearer a nature is to God, the more is the Divine dignity expressed in it."

"Now it belongs to the Divine dignity that God should move, incline and direct all things, He being neither moved nor inclined nor directed by anyone. Hence the nearer a nature is to Him, the less is it by Him inclined, and the more is it adapted to incline itself. Insensible nature, therefore, being, by reason of its materiality, the furthest removed from God, is inclined indeed to some end, but has not in itself anything to incline itself. It has only the principle of inclination, as is evident from what has been said. Now, though a sensitive nature, as being nearer to God, has in itself something that inclines, viz., the appetibile apprehended, yet the inclination itself is not in the power of the animal inclined, but is determined from without, (i.e. by God). Animals, when they see a desirable thing, cannot help wishing for it, because they have no dominion over their own inclination. Wherefore they may be said to be drawn, rather than to draw themselves, as St. John Damascene says: which is because the sensible appetitive power has a corporeal organ, and therefore is near to the dispositions of matter and of things corporeal, so as to be moved rather than move. But the rational nature, which is the nearest to God, not only has inclination to something, as inanimate things have, nor does it only move this inclination by mere extrinsic determination, as sensitive natures do. Over and above that, it has the inclination itself in its own power, so that it does not necessarily incline towards the appetibile apprehended, but may either incline or not incline: so that its inclination is not determined for it by another, but by itself.{3} And this belongs to it not because it uses no corporeal organ, but because, withdrawing from the nature of a moveable thing, it approaches the nature of a mover and operator. Nothing can determine its own inclination to an end without knowing the end and the aptitude of the means to attain it (habitudinem finis in ea quae sunt ad finem); which reason alone can do. And therefore this appetite, being not determined necessarily by others, follows the apprehension of reason; wherefore the rational appetite, which is called the will, is different from the sensitive appetite."{4}

From this evidence we gather the noblest conceptions about the wondrous unity of corporeal substances, which in their multiplex tendencies imitate variously the Divine perfections; but we must content ourselves now with noting some deductions that serve our present purpose.

Since the order and beauty of the corporeal universe depends on the reciprocal operations of corporeal substances, and these cannot operate without approaching, therefore to all of them an inner inclination is given, called universal gravitation, by which they tend towards each other. As this inclination is solicited (through some medium) by the interchanging action of corporeal substances, so this solicitation receives the name of universal attraction. Thus these substances have a principle by which (quo) they attract, and a principle by which they go to the attracting body, and an inner disposition suited for that effect.

But if there were no other than universal gravitation, those substances, though tending by mutual approach to form an aggregate in the universe, would be wanting in that distinction and order from which the beauty of each part results. Moreover the formation of new substances requires that substances should unite and be transformed into other substances of a determinate nature, and that these be disposed for mutual attraction, so as to operate specially on each other. Universal attraction and gravitation would not be sufficient in the universe. Particular attractions and particular gravitations are required, which, though reducible to the same genus, constitute different species. From this chemical affinities arise, and the inclining of certain substances to be united with other determinate substances. Such affinities and inclinations must also have a true cause in the inner disposition of substances, which are therefore said to have affinity to each other, like what we have seen in universal gravitation -- like, but not equal, because we cannot speak of species identically, as we speak of a genus. As these dispositions, by the bye, are qualities, no wonder if they diminish or fail without a change of the whole substance into another substance. Lastly, it follows, from what we have said, that universal attraction and gravitation being generic and therefore common to all corporeal substances as such, it will outlast their transformation. So that, as the weight of bodies results from gravitation, the weight of the elements will remain in the compound that results, and the weight of the compound will remain in the elements into which it is resolved. Here we must be understood to mean the absolute weight; for we should have to speak otherwise of the specific weight, which is also in proportion to the real volume. Hence, if the specific weight is greater or less in the compound than the absolute weight that was in the elements, greater or less will the absolute weight be; e converso. We have now to speak of physical laws, which may be considered as a corollary of the doctrines hitherto explained.

{1} Quaest. Disp., De Veritate, Q. xxii. a. 3.

{2} Quaest. Disp., De Veritate, Q. xxii. a. 1.

{3} Hence whilst human will is evidently determined by God to the universal bonum, it is equally clear that the will freely determines itself to the particular objects in which the mind recognizes a participation of that universal bonum. And this reconciles the Divine impulse with human freedom as taught by St. Thomas.

{4} Quaest. Disp., De Veritate, Q. xxii. a. 4.

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