PHILOSOPHICAL TERMINOLOGY.
III.
To The Editor Of The Catholic World:
In my last letter, while criticising an incorrect definition of the word act, I made the remark that "the gravity of bodies is not a power, as some unphilosophical scientists imagine."[268] When writing these words, I had to confine myself to a mere statement of the scientific error; but it occurs to me that in an age in which most of the so-called men of science are so little acquainted with philosophy as to mistake effects for causes, and yet so proud of their achievements as to aspire to the leadership of the public mind, some precautions must be taken, lest our philosophical terminology be infected with such improprieties as are now too leniently tolerated in the language of science. It is the abuse of one word that does the greatest mischief in the department of physics. This word is force. Its frequent misapplication tends to confound and falsify the whole doctrine of physical causation. It is therefore of great importance, even in a scientific point of view, to determine within what limits the use of such a word should be restricted in accordance with the laws of philosophical terminology. Such is the main object of my present communication.
The theory of physical causation deals with natural causes, powers, actions, forces, movements, and the results of movements. When these terms are properly defined, all relations between agents and patients, between causes and effects, and consequent phenomena, can be easily expressed with philosophical precision; but when the causes, the powers, the actions, and the movements themselves are all confounded under one common name of force, as it is now the fashion in the scientific world to do, no one need be surprised if such a course ends in philosophical inconsistencies. To show what great proportions this evil has taken, innumerable passages of modern writers might be adduced. But, not to perplex the reader with conflicting quotations from different sources, I will give only a few extracts from one of the best representatives of modern science. I have before me the Correlation of Physical Forces, by Mr. Grove. It is a well-known little work, much esteemed by physicists, and one which certainly transcends the average merit of many modern productions of the same kind. Now, what is Mr. Grove's notion of cause, of force, of power, as compared with one another and with the phenomena of nature? The following passages will show. He says:
"In each particular case, where we speak of cause, we habitually refer to some antecedent power or force; we never see motion or any change in matter take effect, without regarding it as produced by some previous change" (p. 13).
Here force, power, and cause are taken as equivalent; moreover, motion, or a change in matter, is considered as "produced" by a previous change; which implies that a previous change or movement is the efficient cause of a subsequent change or movement. Hence, according to such a terminology, movement, force, power, and cause should be accepted as synonymous. But philosophy cannot admit of such a wholesale confusion.
"A force," says he, "cannot originate otherwise than by devolution from some pre-existing force or forces.... The term 'force,' although used in very different senses by different authors, in its limited sense may be defined as that which produces or resists motion.... I use the term 'force' as meaning that active principle inseparable from matter, which is supposed to induce its various changes" (p. 16).
Here force is again confounded with power and with cause, inasmuch as "active principles" are powers, and "that which produces or resists motion" is a cause. We are told at the same time that the active principle is not a primordial and essential constituent of material substances, but an accidental result of devolution from other active principles residing in other substances. Philosophy cannot admit of such a phraseology; for, as the active principle of a substance is a constituent of its nature, if the active principle of any substance were thus communicated to it by accidental devolution, such a substance would have no definite nature of its own, and would be nothing; and, in spite of this, it would also be capable of becoming anything, according as its active principle might originate from different pre-existing forces. Now, we know that the first elements of any given substance have a definite nature, and a definite active principle independently of devolution from other substances; and that, according to the results of a constant and universal experience, they are not liable to exchange their nature for anything else, but keep it permanently and unalterably amidst all the vicissitudes brought about by the interference of surrounding bodies. It is, therefore, plain that the "active principle inseparable from matter" cannot originate in devolution from other pre-existing forces. But let us proceed.
"The position which I seek to establish in this essay," says Mr. Grove, "is, that the various affections of matter which constitute the main object of experimental physics—viz., heat, light, electricity, magnetism, chemical affinity, and motion—are all correlative, or have a reciprocal dependence; that neither, taken abstractedly, can be said to be the essential cause of the others, but that either may produce, or be convertible into, any of the others" (p. 15).
Every one, of course, will admit that heat, light, electricity, etc., are "correlative." I also admit that they are not "essential causes" of one another; but the fact is that they are no causes at all; since heat, light, electricity, etc., are only modes of motion and "affections of matter," as the author acknowledges, and are therefore to be considered as mere phenomena or effects, of which the one can be the condition, but not the cause, of the other. I know that the popular language admits of such expressions as "heat causes dilatation," "light causes an impression on the retina," "chemical affinity causes combination," "movement causes a change of place." But these and other similar expressions, though used by scientific writers, and even by philosophers, are by no means philosophically correct. We shall see presently that substances alone have efficient powers, and therefore no mode of being and no affection of matter can display efficient causality. Hence light, heat, electricity, and the rest, are neither efficient causes nor efficient powers; and, inasmuch as they are affections of matter, they cannot even be called forces in a philosophical, but only in a technical, sense, as we shall explain hereafter.
As to the mutual "convertibility" of these various affections of matter into one another, I would observe that, although the expression may be correctly understood, yet, as interpreted by Mr. Grove and by other physicists, it cannot be admitted. What do we mean when we say that progressive motion, for instance, is converted into heat? We mean that in proportion as the progressive movement of a body is resisted and extinguished, a correspondent amount of heat, or of molecular calorific vibrations, is produced by mutual actions and reactions. In this sense the conversion of one mode of being into another is perfectly admissible, no less indeed than the passage from a state of rest to one of movement. But Mr. Grove does not understand it so. He thinks that the progressive movement of a body is never extinguished, but only transformed by subdivision into molecular calorific vibrations; and, therefore, that the same accidental entity which was to be found in the progressive movement is still to be found identically, though subdivided, in the calorific motion. Let us hear him:
"It is very generally believed that, if the visible and palpable motion of one body be arrested by impact on another body, the motion ceases, and the force which produced it is annihilated. Now, the view which I venture to submit is that force cannot be annihilated, but is merely subdivided or altered in direction or character" (p. 24). "Motion will directly produce heat and electricity, and electricity, being produced by it, will produce magnetism" (p. 34). "Lastly, motion may be again reproduced by the forces which have emanated from motion" (p. 36).
Such is Mr. Grove's theory of the "convertibility of forces." It is nothing but a wrong interpretation of the old theory of the "conservation of vis viva" by the modern conception of "potential energy," which admits "forces stored up" in bodies, and ready to show themselves in the form of velocity, heat, light, or any other kind of movement. This notion and others of a like tendency constitute the marrow of the new physical theories, and are the pride of our men of science. Let us hope that a time will come when these able men will see the vanity of such fashionable doctrines, and blush for their adoption of them in their scientific generalizations.
The conservation of vis viva is, within certain limits, that is with regard to ponderable bodies impinging on one another,[269] an established fact; but its interpretation as given by advanced physicists is a huge blunder. "It is very generally believed," says Mr. Grove, "that if the visible motion of one body be arrested by impact on another body, the motion ceases." Of course, it is believed; and, what is more, it is demonstrably true, whatever Mr. Grove may say to the contrary. Yet it is not true, nor is it very generally believed, that "the force which produces it (the motion) is annihilated." When the movement of a body is arrested, its velocity is extinguished; but that velocity was not the force which produced the movement. When a stone falls to the ground, its movement is produced, not by its velocity, but by the action of the earth on it. Velocity is only the formal principle of movement, and is itself included in movement as a constituent, not as an efficient power. To say that velocity produces movement is, therefore, to confound formal with efficient causation, and to admit that movement produces itself. This is one of the conclusions for which I hope, as I said, physicists will blush hereafter.
But the force, we are told, "is not annihilated, but merely subdivided or altered in direction or character." This cannot be. The word force here means a quantity of movement, which is nothing but the product of the velocity into the mass of the body. Now, the velocity of a body is not subdivided when the movement is arrested, but is really extinguished. I say extinguished, not annihilated; because annihilation, as well as creation, regards substances, not accidents. Velocity is an accident; it is therefore neither created nor annihilated, but originates in a determination produced by an agent, and ends by exhaustion or neutralization under the influence of an antagonistic agency. I say, then, that the movement of the body, though not annihilated, is extinguished and not subdivided. It is impossible to conceive of divisions where there is nothing divisible. On the other hand, nothing is divisible which has no extension and no material parts. Now, where are the material parts or the extension of velocity? Velocity in each primitive particle of a body is a simple actuality, which can increase or decrease by degrees of intensity, but cannot be taken to pieces in order to be apportioned among the other particles of the body, and therefore the pretended subdivision of velocities is a mere absurdity.
Nor does it matter that force can be "altered in direction or character." We must not forget that force is here a sum of velocities, and accordingly cannot change direction or character unless such velocities are intrinsically changed. But they cannot be changed with regard to either character or direction without some new degree of velocity being produced or extinguished by some efficient cause. For the character of velocity is to actuate the extension of the movement in proportion to its own intensity. This, and no other, is its character; and, therefore, velocity cannot be altered in character without its intensity being increased or diminished by action. And the same is to be said of the change of direction, which cannot be conceived without action. Now, if action can modify motion, and diminish to any extent its velocity, it remains for our scientists to explain how a certain action cannot stifle movement and velocity altogether.
They will say that the "indestructibility of force" is the only hypothesis consistent with the theory of the conservation of vis viva, and consequently that the two must stand or fall together. But the truth is that the conservation of vis viva needs no such hypothesis, since it depends on a quite different principle, viz., on the equality of action and reaction.
When two billiard-balls impinge on one another, they act and react. Their molecules urge one another (by their mutual actions of course, not by their velocity), and become compressed. All the work they do up to the maximum of compression is styled action. But reaction soon follows; for, as compression brings the neighboring molecules into an unnatural position where they cannot settle in relative equilibrium, the molecular exertions tend now to restore within the bodies the original molecular distances; which work of restoration is properly called reaction.[270] And since reaction must undo what the preceding action had done, hence the amount of the reaction must equal the amount of the action, and thus no energy is lost; for the same quantity of movement is produced in one ball as is extinguished in the other.
I do not wish to enlarge on this topic, which is of a physical rather than metaphysical nature. I only repeat that the mistake of our physicists lies in supposing that the quantity of movement which is lost by one body still exists in nature, and passes identically into another body; whilst the fact is that the quantity of movement lost by the first body is altogether extinguished, and the quantity acquired by the second body is a new production altogether. To send an accidental mode, such as velocity, travelling about from one substance to another without support, as an independent and self-sufficient being, may be a bright device of modern progress; but when the time comes for repenting of other scientific blunders, this bright delusion will, I am sure, be reckoned among the most grievous philosophical sins that science will have to regret and to atone for in sackcloth and ashes.
These remarks go far to show that the terminology of our modern scientists concerning physical causation is philosophically incorrect. I have more to say on this same subject; but to make things plainer I wish to give beforehand what I consider to be the true distinction between cause, power, action, and force, as implied in the causation of natural phenomena. To do this in the most simple and intelligible manner, I lay down the following propositions:
I. It is a principle philosophically certain that the substance of all natural things has been created by God for his extrinsic glory—that is, for the manifestation of his power and other perfections. Accordingly, every created substance has received a natural aptitude and fitness to manifest in some manner and in some degree the power and perfection of its Creator.
II. Therefore, every creature naturally, per se, not accidentally, but by the very fact of its creation, is destined to act; for manifestation is action, and consequently possesses permanently and intrinsically such an active power as is proportionate to the kind and degree of the intended manifestation. In other terms, every created substance is destined to be the efficient cause of determined effects.
III. The power of created substances is finite, and its exertion is subject to definite laws. All finite power, according as it is exerted under more or less favorable conditions, gives rise to effects of greater or less intensity. Hence different effects may proceed from one and the same cause, and equal effects from different causes, acting under different conditions.
IV. The exertion of power is called action, and its intensity, in the material world, depends on the distance of the agent from the patient.
V. The amount of the exertion, or the quantity of the action, is measured by its true effect, which is the only true exponent or representative of the degree of the exertion; for, all matter being equally indifferent to receive motion, the amount of its passion must always agree with the amount of the action received; and thus the one is the natural and necessary measure of the other.
VI. The amount of the exertion, as measured by the effect it is able to produce, is what in the scientific language can be styled force properly.
VII. The amount of the effect, as measuring the amount of the exertion from which it arises, or by which it is neutralized, is again called force, but improperly, and only in a technical sense, as it is in fact a mere measure of force.
These propositions are so logically connected with one another that, the first of them being admitted, all the others must follow. I might, therefore, dispense with all discussion with regard to them; yet, to help the scientific reader to form a philosophical notion of forces, I will endeavor to throw some additional light on my sixth and seventh propositions.
And, first, I observe that since forces can only be measured by their effects, the mathematical expression of a force always exhibits the quantity of the effect which such a force is competent to cause; and as such an effect is a certain quantity of movement, hence forces are mathematically expressed in terms of movement. So long as physicists preserved their old philosophical traditions, a distinction was kept up between force and movement. A quantity of movement was indeed called a force, inasmuch as it was the true measure of the action from which it had originated, or by which it could be destroyed; but such a force was not confounded with the action itself. The action was called vis motrix, a motive force, whilst the quantity of movement was called vis simply, and was not considered as having any efficient causality. Thus before Dr. Mayer's invention of "potential energies," the word force was used with proper discrimination: 1st, as a quantity of action actually producing movement; 2d, as a quantity of action actually opposed by a resistance sufficient to prevent the production of movement; 3d, as a quantity of movement and a measure of action.
A quantity of action followed by movement was called a dynamical force, and was measured by the quantity of movement imparted in the unit of time. Its mathematical expression in rational mechanics was, and is still, a differential coefficient of the second order representing the product of the mass acted on into the velocity which the action, if continued for a unit of time, would communicate to it. As instances of dynamical force, we may mention the action of the sun on the planets, of the planets on their satellites, of the earth on a pendulum, on a drop of rain, etc.
A quantity of action not followed by movement was called a statical force, and was measured by the quantity of movement into which it would develop, if no obstacle existed. Its mathematical expression in rational mechanics is a differential coefficient of the first order representing the product of the mass, whose movement is neutralized into its virtual velocity. By virtual velocity we mean the velocity which the mass would acquire in a unit of time, if all resistance to the movement were suddenly suppressed. As instances of statical force, we may mention the action of a weight on the string from which it hangs, or on the table on which it lies.
A quantity of movement, or the dynamical effect of all the actions to which a body has been subjected for any length of time, was called a kinetic force. As kinetic forces cannot be destroyed except by actions producing equal and opposite quantities of movement, hence every kinetic force can be taken as a measure, not only of the amount of action from which it has resulted, but also of the amount of action by which it can be checked. The mathematical expression of a kinetic force is the product of the moving mass into its actual velocity. As instances of this force, we may mention the momentum of a cannon-ball, of a hammer, of wind, falling water, etc.
To obviate the many abuses which this notion of kinetic force has engendered, and to cut the ground from under the feet of those blundering theorists who reduce all forces to movement, it is important to remark that kinetic force could be defined as "that quantity of action which a moving body can exercise against an obstacle until its velocity is exhausted." This definition would change nothing in the mathematical expression of kinetic forces; for the quantity of the action which a moving body can exercise against the obstacle is exactly equal to the quantity of movement, or momentum, by which the body is animated. The only change would be in the terminology, which, instead of technical, would become philosophical. As instances of kinetic force thus defined, we might mention the quantity of action of a cannon-ball, of the hammer on the anvil, of the wind on the sails, etc.
The division of forces into dynamical, statical, and kinetic has been long recognized by all competent judges as very good and satisfactory. But our men of progress, in the innocent belief that, before they appeared on the scene, everything in this world was darkness, have changed all that. All forces are now stated to consist in nothing but "mass animated by velocity." Dynamical forces are rejected, it would seem, because they imply what modern science cannot, or will not, understand—i.e. real production (they call it creation) of movement. On the other hand, statical forces are not masses animated by velocity, and thus are set aside because they originate no real movement. Such is the consistency of our progressional friends.
Yet so long as all effect will need a cause, there can be no doubt that statical forces must be real forces. Two weights balancing one another at the ends of a lever certainly act on one another, as every one must admit who observes the change produced by taking away one of the two. A weight which actually prevents another weight from falling surely exerts a positive influence on it, and therefore displays power and brings forth an amount of action. So also, when a weight is at rest on a table, gravity does not remain dormant with regard to it, but urges it toward the table with unyielding tenacity. Hence the table must continually react in order to keep the body at rest. It is evident, therefore, that the weight, while at rest on the table, exerts its powers and is engaged in real action; for nothing but real action can awaken real reaction. Again, when we try to raise a weight, we feel that we must overcome a real resistance; and when we support a weight, we feel its action upon our limbs. Hence pressure is a real force, though it be not mass animated by velocity; and the same is evidently to be said of traction, torsion, flexion, etc. It is, therefore, impossible to ignore statical forces.
That dynamical forces are likewise indispensable in science I think it would be quite superfluous to prove. Rational mechanics is wholly based upon them, and no phenomenon in nature can be explained without them. If modern science finds it difficult to understand the production of local movement, let her consider that, after all, it would be less damaging to her reputation to confess her philosophical ignorance than to deny what all mankind hold and know to be a fact.
From this short discussion we may safely conclude, with the old physicists, that there are in nature dynamical and statical as well as kinetic forces, and that the word force should be uniformly used in philosophy as expressing a quantity of action measured by the quantity of its effect, or by something equivalent. But we have not yet done with our advanced theorists.
It is curious that, after having reduced all forces to "mass animated by velocity," they have not hesitated to introduce into science a force which is neither mass animated by velocity nor a common statical force, but something quite different, to which they gave the name of "potential energy." The first to imagine this spurious force was, if I am not mistaken, the German Dr. Mayer, one of the great leaders of modern thought, who, considering that a body raised from the floor would, if abandoned to itself, fall down and acquire a momentum calculated to do an amount of work, conceived the raising of the body as equivalent to a communication of latent energy destined to become visible at any time in the shape of movement as soon as the body is left to itself. Such an energy, as still unevolved, was called "potential energy."
"If we define 'energy' to mean the power of doing work," says a well-known English professor, "a stone shot upwards with great velocity may be said to have in it a great deal of actual energy, because it has the power of overcoming up to a great height the obstacle interposed by gravity to its ascent, just as a man of great energy has the power of overcoming obstacles. But this stone, as it continues to mount upwards, will do so with a gradually decreasing velocity, until at the summit of its flight all the actual energy with which it started will have been spent in raising it against the force of gravity to this elevated position. It is now moving with no velocity—just, in fact, beginning to turn—and we may suppose it to be caught and lodged upon the top of a house. Here, then, it remains at rest, without the slightest tendency to motion of any kind, and we are led to ask, What has become of the energy with which it began its flight? Has this energy disappeared from the universe without leaving behind it any equivalent? Is it lost for ever, and utterly wasted?... Doubtless the stone is at rest on the top of the house, and hence possesses no energy of motion; but it nevertheless possesses energy of another kind in virtue of its position; for we can at any time cause it to drop down upon a pile, and thus drive it into the ground, or make use of its downward momentum to grind corn, or to turn a wheel, or in a variety of useful ways. It thus appears that when a stone which has been projected upwards has been caught at the summit of its flight and lodged on the top of a house, the energy of actual motion with which it started has been changed into another form of energy, which we denominate energy of position, or potential energy, and that, by allowing the stone again to fall, we may change this energy of position once more into actual energy, so that the stone will reach the ground with a velocity, and hence with an energy, equal precisely to that with which it was originally projected upwards."[271]
Such is the theory. It is scarcely necessary to say that the whole of it is a delusion. First, the velocity imparted to the stone is not a working power, but only a condition for doing work, as I shall presently show; and, therefore, it cannot be styled "energy."
Secondly, when the stone is caught at the summit of its ascent, and (according to the strange phrase of the author) is moving with no velocity, it possesses nothing more than it possessed when lying on the ground. Its elevated position is only a new local relation, which confers no power, either actual or potential. It is indeed possible to let the stone drop down; but then its fall will be due to the action of the earth, and consequently to extrinsic causation, not to anything possessed by the stone on account of its elevated position.
Thirdly, the words "potential energy" cannot be coupled with one another without absurdity; for "energy," according to all, means power to act, whilst "potential" means liability to be acted on. Hence "potential energy" would mean either a power to act which is ready to be acted on, or a power which is to be acquired by the body through its being acted on. The first alternative confounds act with passive potency, and action with passion; the second assumes that the velocity to be acquired by the body is a real working power, which it is not.
Fourthly, it is against reason to admit that "the energy of actual motion is changed into another form of energy." For where is the causality of the change? The only causality concerned with the modification of the upward movement of the stone is the action of gravity; and this, being directly antagonistic to the ascensional velocity, tends to destroy, not to transform, it.
Fifthly, a stone created originally on the brink of a precipice would be ready to fall into it, although it has never been thrown up; on the contrary, a stone thrown up to such a height as to reach the limits of the moon's effectual attraction would never come down again, notwithstanding the enormous amount of pretended "actual energy" expended in the mighty ascent. Hence the upward flight has nothing whatever to do with any so-called "potential energy." It is, therefore, a gross delusion to hold that by allowing the stone again to fall, "we may change the potential energy into actual energy," it being evident that the actual velocity of the falling stone is not a result of transformation, but the product of continuous action.
We cannot, then, adopt the phrase "potential energy" in metaphysics. The phrase means nothing; for there is nothing in nature which can be designated by such a name. Energy is synonymous with power; and power cannot be in a potential state. To be in potency to receive any amount of velocity is not energy, but passivity. On the other hand, the power of doing work is not a mere force, as assumed by the modern theory, but is something much higher and better. Forces are only variable quantities of action; the power, on the contrary, in one and the same body is always the same, and yet is competent to do more or less work, according as it is exerted under more or less favorable conditions. The stone that is hurled against a pane of glass exerts, in breaking it, the very same power which it exerted before being hurled; only the conditions of the exertion are quite different, inasmuch as its velocity brings it against the glass at such a rate that, before its movement can be checked by the action of the glass, the stone has time to outrun it, dashing it to pieces. Yet it is by its action, not by its velocity, that it does such a work. Of course, its action is proportional to its velocity, and its work is proportional to the square of its velocity; and thus the velocity serves to measure both the work and the action, but it does not follow that the velocity is the active power. Velocity is an accidental mode of being; and nothing accidental is active. This important philosophical truth can be easily established as follows:
In all things the principle of being is the principle of operation, as philosophers agree; whence the axioms, "By what a thing is, by that it acts," and "Everything has active power inasmuch as it has being." Now, all substance has its being independently of accidents; therefore, all substance has its active power independently of accidents. On the other hand, accidents give to the substance a mode of being, and nothing more; therefore, they also determine its mode of acting, and nothing more. But as to be in this or that state presupposes being, so also to have a power ready to act in this or that manner presupposes power. Hence no accident gives active power to the substance of which it is the accident; or, in other terms, accidents are nothing more than conditions determining the mode of application of the active powers that pre-exist in the substance.
Again, all natural accidents[272] are reducible to three classes; as some of them are accidental acts produced by some agent and passively received in some subject, others are intrinsic modes of being resulting from the reception of such accidental acts, and, lastly, a great many are mere relativities or relative modes. Now, that relativities can act no one has ever pretended to assert. That intrinsic modes of being can act, is implicitly assumed by all who consider velocity as an active power; for velocity is an intrinsic mode of being. Yet if we ask them whether the existence of things is competent to act, they will certainly answer no; and they will be right. But, I say, if existence cannot act, still less can a mere mode of existing act. For a mode of existing is a reality incomparably less than existence itself. Accordingly, since they concede that not the existence, but the thing existing, is a principle of action, they must also à fortiori concede that the thing modified, and not its mode, is a principle of action. Finally, with regard to the accidental act, it is evident that its reception in the substance cannot impart to it any new activity, since its formal effect simply consists in a new mode of being, which, as we have just seen, is not active. It is clear, then, that no natural accident has active power.
Omitting other reasons drawn from theoretical considerations, and which might be usefully developed in special metaphysics, I will only add an à posteriori proof, which physicists will probably find more congenial to their habit of thought. It consists in the fact that bodies act on one another without being animated by velocity, or without their velocity having any share in the production of the effect. Thus a book at rest on a table acts on the table; and a liquid, or a gas, at rest in a jar acts on the jar. On the other hand, the earth, though not at rest, attracts bodies, not by its diurnal rotation or by its annual revolution, but by a power dependent only on its mass; and the same is to be said of the sun and the planets. This shows that the power from which the motive action of bodies proceeds is not their velocity; whence it follows that velocity is only an affection of bodies, and has no bearing upon the active powers of the same, but only on the mode of their application. Now, since all the accidents which have been supposed to involve active power can be resolved into kinds of movement, it must be owned that such accidents have no real activity; for all kinds of movements consist of velocity, and velocity does not act.
Hence, whatever scientists may say to the contrary, heat, light, electricity, etc., are not efficient powers, but modes of movement, on which the mode of acting of bodies depends. When heat was thought to be a subtle imponderable substance, philosophers could consistently call it an efficient power; but since it is now decided that heat is only "a mode of motion," how can we still attribute to it what is the exclusive property of substances? If heat is only a mode of motion, a bar of iron, when hot, has no greater powers than when cold; it has only a greater movement. So also, if light is only a mode of motion, luminiferous æther has no greater power when undulating in the open air than when at rest in a dark room. In the same manner air, when perfectly still, has the same powers as when actually propagating any variety of sounds. When, therefore, physicists speak to us of such movements as powers, let us not be imposed upon by their phraseology, if we wish to be consistent in our reasonings, and avoid useless and troublesome disputes.
Yet it was to be expected that our physicists in their technical language would confound heat, light, and other modes of movement with forces and powers. The correlation between such movements and the actions of the bodies subjected to them is, in fact, such as to allow of the former being taken for measure of the latter. Thus a given amount of mechanical action may give rise to a definite amount of heat, and vice versa; hence the one can be technically considered as the equivalent of the other, inasmuch as the one is the measure of the other. But does it follow that action and heat belong to the same category? Certainly not. It is not the action itself, but its mechanical effect, that should be taken as the true equivalent of the heat generated. And when we are told that "heat is expended in generating mechanical movement," we must not fancy that calorific movement causes another kind of movement, as the phrase seems to imply, but only that, while the calorific movement is diminished by a given cause, the same cause generates the mechanical movement. We should always bear in mind that the language of modern science, though correctly expressing the correspondence of effects to effects, is very far from expressing as correctly the relation of effects to causes. Physicists should learn to distinguish between efficient causes and conditions determining the mode of their causation. Heat is one of such conditions, and to call it a force is to endow it with efficient causality; for the term force always conveys the idea of causation. They should either cease to describe heat as a force, or, if this cannot be done, explain more explicitly than they do the technical restrictions modifying the philosophical meaning of the word. We can hardly expect that they will follow our advice; but, at any rate, it is to be hoped that philosophers at least will take care to follow it, and guard against the corruption of their own terminology.
Besides heat, light, electricity, and magnetism, there are many other modes of being technically called forces. Centrifugal force is one of them; for, in fact, centrifugal force, in spite of the name, is nothing more than "that quantity of movement which is extinguished by centripetal action in the unit of time."
The force of inertia—vis inertiæ—is another technical or conventional force. For it is plain that inertia cannot act; and thus it is impossible to conceive any true force of inertia. But, technically, vis inertiæ means "the quantity of the effort by which a body, when enduring violence from without, resists compression, traction, or any other alteration of its molecular structure." This effort proceeds, not from inertia, but from the active powers residing in the molecules of the body; and yet it has received the name of vis inertiæ, because it develops itself in the lapse of time during which the body, inasmuch as inert—i.e. incapable of leaving its place before the whole mass has acquired a common velocity—is still loth to start, and thus compelled to struggle against the invading body. Philosophers, by keeping in sight this definition of vis inertiæ, will be able to solve many sophisms of modern scientific writers.
Again, the weight of a body is called a force, and is represented by the product of the mass of the body into a velocity which it has not, but which it would acquire through the action of gravity in a second of time, if it were free to fall. If the mass be called M, and the velocity which it would acquire g, the product Mg will represent the weight of the body. Now, when the body is at rest on a table, the pressure exercised by it on the table is said to be Mg. Does this mean that the weight of the body acts on the table? Not at all; for the body does not act by its weight, which is not an active power. The truth is that the table by its resistance prevents the body from acquiring the momentum Mg; and since this resistance of the table must be equal to the pressure exercised on it by the body, hence the pressure itself is also equal to Mg; and thus a true force—a quantity of pressure—is technically identified with the weight of the pressing body. This identification tends to give a false idea of the nature of the fact, and therefore should be carefully avoided in philosophy.
Modern physicists have laughed at a philosopher of the old school (Arriaga), who, as late as 1639, "was troubled to know how, when several flat weights lie upon one another on a board, any but the lowest should exert pressure on the board." It would have been more prudent on their part to ask themselves whether the question was one which the modern school could answer at all. If we ask how two equal weights can exert equal pressures on the board from unequal distances, what can they answer? If they wish to be consistent with their notions, they can only answer that "the actions are transmitted from one weight to another till they meet the board." Now, this is a great philosophical blunder; for actions are accidents, and therefore cannot travel from one subject to another. Neither action nor active power are ever transmitted; not even movement is properly transmitted, but only propagated by a series of successive exertions from molecule to molecule. Were we to admit in philosophy any such transmission, we would soon be entangled in innumerable contradictions.
Mechanical work also is often styled a force, though it is nothing but the process by which a force is exhausted. The notion of work is very simple. A body moving through space against a continuous resistance is said to do work. Work is therefore so much the greater according as a greater mass measures a greater space under a greater resistance; and thus the work which a given body can perform may be represented by the product of three factors, viz., the mass, the mean resistance, and the space measured. This is the philosophical and analytical expression of work; and mathematicians show that this expression in all cases (viz., whether the resistance be constant or variable) is equal to half the product of the mass into the square of its initial velocity. Now the question comes: Is work a force? It is not difficult to anticipate the answer. Since the adoption of the so-called "living forces," or vires vivæ, of Leibnitz, physicists have called vis viva the sum of the works of two conflicting bodies; and consequently the work done by either of the two was said to be one-half of the vis viva. But, with all the respect due to the memory of Leibnitz, I would say that neither the work nor the so-called vis viva is a force in the philosophical sense of the word. When a mass, M, animated by a velocity, V, encounters a resistance and begins its work, its momentum is MV. This momentum, while the work is being done, is gradually reduced till it is finally destroyed by the resistance. The resistance is, therefore, equal to the momentum MV. But the resistance, according to the law of impact, is always equal to the exertion of the impinging body. And therefore the amount of the exertion of the impinging body is also equal to MV; that is to say, the force by which the work is done is an ordinary dynamical force represented by the usual dynamical momentum, and not by the amount of the work done.[273]
And here I must close this rather long excursion into the field of mechanics. But I cannot conclude without calling the reader's attention to the reckless tendency of the phraseology which I have above criticised. It seems as if the object of a class of scientific writers in these late years has been to banish from science all secondary causes, no doubt as a preliminary step (in the intention of the most advanced among them) for the banishment of the First Cause itself. The words cause, power, force, and others of the same kind, have indeed been maintained, as they could not be easily dispensed with; but they receive a new interpretation: they have become "kinds of motion," and have been identified with the phenomena—that is, with the effects themselves. Thus "movement" is now everything; its boasted "indestructibility" makes it independent of all secondary causes; and we are told that the existence of "essential causes" can no longer be proved by the phenomena, and that "science" has the right to reject them as metaphysical dreams. Let us hear Mr. Grove again:
"Though the term (force) has a potential meaning, to depart from which would render language unintelligible, we must guard against supposing that we know essentially more of the phenomena by saying that they are produced by something, which something is only a word derived from the constancy and similarity of the phenomena we seek to explain by it" (p. 18).
And again: "The most generally received view of causation—that of Hume—refers to invariable antecedence—i.e. we call that a cause which invariably precedes, that an effect which invariably succeeds" (p. 10).
And again: "It seems questionable not only whether cause and effect are convertible terms with antecedence and sequence, but whether, in fact, cause does precede effect.... The attraction which causes iron to approach the magnet is simultaneous with, and ever accompanies, the movement of the iron" (p. 13). Yet he adds: "Habit and the identification of thoughts with phenomena so compel the use of recognized terms that we cannot avoid the use of the word 'cause,' even in the sense to which objection is taken; and if we struck it out of our vocabulary, our language, in speaking of successive changes, would be unintelligible to the present generation" (ib.)
And lastly: "In all phenomena, the more closely they are investigated, the more are we convinced that, humanly speaking, neither matter nor force can be created or annihilated, and that an essential cause is unattainable. Causation is the will, creation the act, of God" (p. 218).
It is not Mr. Grove alone that entertains such views; I might quote other English authors, and many German, Italian, French, and American writers whose opinions are even more extravagant. But a theory which pretends to ignore efficient causality, no matter how loudly trumpeted by scientific periodicals, no matter how pompously dressed in scientific books, no matter how constantly inculcated from professorial chairs, in the long run is sure to fail. It bears in itself and in its very phraseology its own condemnation. It is vain to pretend to explain away its inconsistencies by alleging that "the habits of the present generation compel the use of recognized terms;" the simple truth is that the abettors of modern thought reap in their inconsistencies the reward of their vanity. Mankind will never consign created causality to the region of dreams, and we would remind our scientific friends who have not received a thorough philosophical training of the old adage, "Let the cobbler stick to his last."
A Friend of Philosophy.
FOOTNOTES:
[268] Catholic World, November, 1873, page 187.
[269] This limitation is necessity. A stone thrown up vertically soon loses its vis viva without compensation. The case is one in which there is no impact. An imponderable body, as luminiferous æther, if it forms, as it is most probable, an unresisting medium, acquires vis viva without interfering with the vis viva of the celestial bodies.
[270] Physicists sometimes give the name of action and reaction to the opposite efforts of two conflicting bodies. But, properly speaking, the two efforts are two actions; the reaction only begins at the end of compression, and takes place mostly within each body separately.
[271] Balfour Stewart, Lessons in Elementary Physics, P. 101.
[272] I say natural accidents. The species of bread and of wine in the Holy Eucharist are supernatural accidents, and have no less active power than the substances themselves. The reason is that they imply in their constitution "the act and the activity of the substance"—actum et vim substantiæ—as S. Thomas teaches, and "all that which belongs to matter"—omne illud quod ad materiam pertinet. See the Summa Theol., p. 3, q. 77, a. 5.
[273] In the New American Cyclopædia, edited in 1863 (v. Mechanics), after the statement that "to overcome all the inertia of a body moving with a certain velocity, or to impress on it at rest such a velocity, the same whole quantity of action must in either case be exerted and expended upon the body," we are given to understand that this quantity is equal to half the product of the mass of the body into the square of the given velocity. From what we have just shown, it is evident that this conclusion is false.