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


DESCARTES asserted that the extension of corporeal substances is per se immutable; nor could he say otherwise after denying substantial forms, in which there is the principle or sufficient reason of the changes of that extension. Now, if we admit the mutability of extension, many facts in nature can be explained, which if it be denied, can only be cut like the Gordian knot. This has, in fact, been experienced by his followers. They could not explain nature consistently with the principle of reasoning; and therefore they must either content themselves with a merely historic or empirical description of nature, or else ignore the principles of reasoning, sometimes admitting action truly at a distance, and sometimes putting forward effects of which, in that system, no proportionate cause could be assigned; which comes to the same thing as denying it. Thus we find that, in the explanation of certain phenomena belonging to proper expansion, some modern scientists, who insist on explaining them by improper expansion only and deny the variability of real volumes, fall back pitiably on ridiculous hypotheses.

Let us here examine the arguments used against us. The first and strongest is a question of reasoning. "Compenetration of corporeal substances," they say, "is repugnant to reason: but without that there cannot be such a thing as proper mutability of extension in a corporeal substance. Therefore, such mutability is repugnant to reason."

In reply we must begin by referring back to what we said{1} about the real and apparent volume of corporeal substances, and why, when treating of the mutability of extension, we must have regard to the former, not to the latter. Even our adversaries admit that a porous body can expand or contract by increase or diminution of its pores, in number or in size: but they deny that a corporeal substance, individual and therefore continuous, can, without having pores, dilate, and while retaining the continuity, become more extended. Thus the question before us is about continuous substances, and not about aggregates of substances.

Having premised thus much, let us come to the major of their argument, viz., that compenetration of corporeal substances is repugnant to reason. What, we ask, do you mean by compenetration "It means," they say, "that one corporeal substance is occupying the same place actually occupied by another." Granted, but we deny its being repugnant to reason. The contradictory alone -- viz., its being and not being at the same time -- is repugnant to reason, because negation only is irreconcilable with affirmation. If therefore compenetration of corporeal substances is repugnant to reason, we should have to say that, in consequence of the compenetration, the substance loses its own essence and ceases to be. Now this cannot be shown, for the essence of a body does not require for itself only a determinate place. The body requires that by reason of its local quantity: and local quantity is an accident not essential to the body but merely natural, and therefore may, by the omnipotence of God, be taken away in a particular case. This is demonstrated by those philosophers who, keeping to the true principles of reasoning, instead of being led by fancy, do not suppose that nothing is true except what is perceived by the senses.

Two bodies therefore can be in the same place, provided that both, or at least one of them, be deprived of local quantity, which has an extrinsic regard to excluding from its own place any other substance that has local quantity. Hence a philosopher may say that compenetration of corporeal substances is not natural, but he cannot say that it is absurd.

It is no answer to say, "I can't conceive such a thing," for a personal want of power to understand can never be a criterion of what is intrinsically repugnant or not repugnant to reason. If so, we should have to reject many indubitable truths as repugnant to reason. The human mind cannot create. It only copies. This means that it cannot well conceive things of which nature does not furnish the species. Therefore of all those things that are beyond or outside the order fixed in nature, and are called supernatural or preternatural, it has but weak and ill-proportioned conceptions, formed almost always by mere analogy. However much we may show by force of argument, that true compenetration is not repugnant to reason, it nevertheless is beyond the order fixed in nature: and therefore it is difficult for the mind of man to form a clear conception of it.

Let us now come to the minor, in which the weak part of the objection is.

"There cannot be such a thing," they say, "as mutability of extension without compenetration." This is false. Compenetration means the simultaneous occupation of one place by more than one body, each of which occupies the whole of it. The mutability implies contraction, which merely requires that one point in the mutable thing should happen to occupy the space left free by another that occupied it before. Thus compenetration differs from contraction as the simultaneous from the successive: and therefore if contraction takes place where there is mutability, compenetration does not. Let us make the conclusion clearer to sense by considering two points, a and b, of one continuous individual substance, as near to each other as you like.

a                         b

a'                 b'

a''             b''

a'''         b'''


Given the gradual contraction of the substance, a and b will tend continually to approach each other but will they ever compenetrate? Certainly not, unless we say that the extension of the substance is not merely diminished by condensing, but quite lost by reducing itself to a mathematical point. Therefore a and b will never compenetrate in p but the distance between them will diminish as according to the measure of its contraction, the distance becoming continually less. This is founded on the principle that, inasmuch as we can conceive a quantity always increasing, though never becoming infinite, so can we conceive a quantity always diminishing but never quite destroyed by mere diminution. What we have said about the substance between these points, a and b, applies equally to any other particle of a continuous substance, and shows that contraction can take place without compenetration.

It is easy therefore to perceive why our opponents persist in saying that mutability of extension implies compenetration. They cannot conceive contraction first, and then local transposition, but only think of the transposition. Thus, for instance, they imagine a continuous spherical substance, which condenses in such a manner that each of its least particles, while remaining as it was, approaches the centre where, with the others, it agglomerates. According to this hypothesis there would be no real contraction of the spherical substance, but only a transposition of each part; which assuredly could not take place without a true compenetration. We say advisedly, "real contraction;" for though the substance would then seem to be more restricted than before, its entity would not be really condensed, but only transposed, as the real surface of a piece of paper is (to make use of a similitude) not diminished by being folded up into a smaller visible size. A particle therefore of the spherical substance cannot, while preserving its former extension, be transferred into the centre without occupying the place of what was there before -- in other words, not without compenetration. But such is not true condensation as taught in the Physical system. According to that each particle of the substance contracts within itself entitatively; so that its whole entity is contracted in one way; and the act of contraction does not require that any particle placed on the surface should break off and transfer itself elsewhere. Nay, as a part of one and the same substance, it will always be conjoined thereto, continuing to form with the whole a continuous substance in its decreasing extension: and thus only it does not keep the space first occupied, but approaches the centre. Therefore, if we conceive in this manner, firstly contraction in the entity itself and then translocation of all the particles that compose it, there is no danger of having to admit true compenetration. If likeness will clear the conception, we can find it in any object that, when seen through a more or less powerful microscope, shows its mass, either more extended without disjunction of parts, or more restricted without compenetration of the parts. Thus the minor proposition of our adversary's argument will not stand, and therefore its conclusion falls to the ground.

And now we come to another difficulty objected against us -- a question of fact, put in this way: "All bodies are porous; and therefore the mutability of extension cannot be admitted. The antecedens is proved from innumerable experiences. The consequens cannot be denied, because in that case all condensation of substance would be effected by diminution of pores, all rarefaction from their becoming enlarged: so that we must reject all mutability of extension as useless."

This argument is clear. Let us examine it. The antecedens may be conceded: for though it is not the result of an adequate deduction, the proofs in favour of all bodies being porous are such and so many, that it may be prudently affirmed as a universal proposition. Not however as it seems to be understood by the followers of the Mechanic system, in which each atom is so isolated that it comes not in contact with its neighbouring atoms at any part. Setting aside other reasons, one fails to see how, according to that opinion, we could possibly affirm and explain the entitative unity of individual substances, especially the animated, which are not mere aggregates.

As to the consequens -- viz., "All bodies are porous, and therefore true mutability of extension must be excluded" -- it will not pass muster in good logic. Granted that by mere mutation of pores the bodies furnished with them can be rarefied or condensed, it does not legitimately follow that all condensation and rarefaction must depend on mutation of pores. In the exposition of the Physical system we admitted that, besides the proper condensation and rarefaction produced by true change of extension, there is the improper, which is produced by a change in the interstices called pores. This distinction was well known among the Scholastics and thereby, as Toledo remarks, the followers of Aristotle were distinguished from those of Democritus and Epicurus. Antiqui non cognoscebant nisi hanc solam (sc. impropriam), cum hoc discrimine, quod illi ponebant intra corpora poros, nos vero plenos subtiliore corpore. Alia est condensatio et rarefactio propria, et haec non fit corporis alterius expulsione vel receptione, sed mutatione ipsius subjecti.{2}

"But there is an axiom," it will be said, "that we must not multiply beings (entia) without necessity: therefore since pores will sufficiently explain the whole thing, away with the mutability of extension." Our answer is this:

Firstly, the argument may be turned against the objector. Given the mutability of extension, the whole thing can be explained, and much better. Therefore, what is the use of supposing such and so many discontinuing pores everywhere? Secondly, our opponent proceeds a posse ad esse, and therefore non valet illatio. It comes to this: "We can explain everything by the theory of pores. Therefore that theory is the only true one, and all others are false."

Nego. There are many phenomena that may be explained in various ways. What right therefore has anyone to insist on one only in this case, excluding mutability of extension, though it serves its purpose well? Thirdly, the axiom quoted against the mutability of extension is not applicable, unless the objector had first proved that Almighty God could not have a suitable end for so endowing corporeal substances. This he would find very hard to do. Fourthly, the pores do not explain everything. This I conclusively proved in another work.{3}

We may therefore legitimately conclude, I think, that the objections cited are not valid. I have not met with others of any weight.

{1} Chapter VI

{2} In IV. Ph., c. 9, Q. xi.

{3} Filosofia Scolastica; Fisica Razionale particolare, Parte i. Lezioni xli. xlii.

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