Popular Science Monthly/Volume 16/March 1880/The Force Behind Nature



SOME thirty years ago, I enjoyed opportunities of discussing with John Stuart Mill (whose younger brother had been for twelve months an inmate of my house) many questions of philosophy in which we both felt the deepest interest. Among these was the Doctrine of Causation set forth in his recently published "System of Logic": "We may define the cause of a phenomenon to be the antecedent, or the concurrence of antecedents, on which it is invariably and unconditionally consequent." I pointed out to my friend that, when this assemblage of conditions is analyzed, it is uniformly found resolvable into two categories, which may be distinguished as the dynamical and the material; the former supplying the force or power to which the change must be attributed, while the latter affords the conditions under which that power is exerted. Thus, I urged, when a man falls from a ladder because (as is commonly said) of the breaking of the rung on which his foot was resting, the real or dynamical cause of his fall is the force of gravity, or attraction of the earth, which pulls him to the ground when his foot is no longer supported; the loss of support being only the material condition or collocation, which allowed the force previously acting as pressure on the rung to produce the downward motion of the man who stood upon it.

To this Mr. Mill's reply was, that the distinction is one of metaphysics, not of logic. I ventured, however, to press on him that, to whichever department of philosophy this point is to be referred, it is one of fundamental importance; that, assuming experience as the basis of our knowledge, we recognize the downward tendency of every body heavier than air, by our sense of muscular tension in lifting it from the ground, or in resisting its descent toward the earth; and that our cognition of force through this form of sensation, being thus quite as immediate and direct as our cognition of motion through the visual sense, ought to be equally taken account of.

The promulgation, about the same time, of the doctrine of the "Correlation of the Physical Forces" by Professor (now Sir William) Grove, and the researches of Mr. Joule on the "Mechanical Equivalent of Heat," seemed to me to bring this view of dynamical causation into yet greater importance, by showing that what is true of that form of force which produces or resists mechanical (or what is now distinguished as molar) motion, may be legitimately extended to those other forms which are manifested in the molecular changes that express themselves in chemical action, or impress us with the sensations of heat, light, etc. Partaking of the general ignorance at that time prevalent in this country of the doctrine of "Conservation of Energy," already promulgated in Germany by Mayer and Helmholtz, I myself endeavored to carry Professor Grove's principle into the domain of biology, by showing that what physiologists had been accustomed to call vital force may be regarded as having the same "correlation" with the various forms of physical force as they have with each other.[1] And in the introduction to the fourth edition of my "Human Physiology" (published in 1853) I thus explicitly defined my position:

When this assemblage of antecedents is analyzed, it is uniformly found that they may be resolved into two categories, which may be distinguished as the dynamical and the material, the former supplying the force or power to which the change must be attributed, while the hitter afford the conditions under which that power is exerted. Thus, in a steam-engine we see the dynamical agency of heat made to produce mechanical power by the mode in which it is applied: first, to impart a mutual repulsion to the particles of water; and then, by means of that mutual repulsion, to give motion to the various solid parts of which the machine is composed. And thus, if asked what is the cause of the movement of the steam-engine, we distinguish in our reply between the dynamical condition supplied by the heat and the material condition (or assemblage of conditions) afforded by the "collocation" of the boiler, cylinder, piston, valves, etc. . . . In like manner, if we inquire into the cause of the germination of a seed—which has been brought to the surface of the earth after remaining dormant through having been buried deep beneath the soil for (it may be) thousands of years—we are told that the phenomenon depends upon warmth, moisture, and oxygen; but out of these we single warmth as the dynamical condition, while the oxygen and the water, with the organized structure of the seed itself, and the organic compounds which are stored up in its substance, constitute the material.

The subsequent general recognition by the scientific world of the "correlation" between the forces of nature (under whatever form expressed) has thus given a breadth of foundation to the dynamical doctrine of causation which it previously lacked; and the doctrine, having been afterward formally developed by Professor Bain, was summarized by J. S. Mill, in the later editions of his "Logic," almost in the very terms in which I had originally propounded it to him in conversation, and had publicly expressed it in the extract just cited: "The chief practical conclusion drawn by Professor Bain, bearing on causation, is that we must distinguish, in the assemblage of conditions which constitutes the cause of a phenomenon, two elements: one, the presence of a force; the other, the collocation or position of objects which is required in order that the force may undergo the particular transmutation which constitutes the phenomenon."[2] Mr. Mill himself still preferred, however, to express the principle in terms of motion rather than in terms of force: "If the effect, or any part of the effect, to be accounted for consists in putting matter in motion, then any of the objects present which has lost motion has contributed to the effect; and this is the true meaning of the proposition that the cause is that one of the antecedents which exerts active force." As this mode of expressing the facts is sanctioned by high authorities at the present time, it may be well for me to explain more fully the basis of my original contention, that our cognition of force is quite as immediate and direct as our cognition of motion; in fact (as I think I shall be able to prove), even more fundamental, inasmuch as our cognition of matter itself is in great degree dependent upon it.

It has been recently well said that "all true science involves both the knowledge of nature and the knowledge of man; it includes the study of mind, as well as of matter. A philosopher may pursue either, but he can have no complete knowledge of what he investigates, without borrowing from the other department of investigation."[3] Many of the Nature-philosophers who affirm that we have no knowledge of anything but the matter and motion which lie within the range of "experience" show themselves very imperfectly acquainted with what "experience" really means; unhesitatingly ranking as actual objective facts their own mental interpretations of the sensory impressions they receive from external objects. Many metaphysicians, on the other hand, have reasoned as if our concern were with mental operations alone, and as if the abstractions in which they deal had an existence per se, without any relation to the phenomena of nature. But, among the ablest thinkers of the present time, there seems to be now a pretty general recognition of the necessity for the replacement of the abstract definitions of metaphysics—so far, at least, as they relate to the external world—by psychological expressions of the modes in which the human ego is affected by its changes. Thus the ordinary metaphysical definition of "matter" is that which possesses "extension." But, for this definition to convey any definite idea to our minds, we must know what "extension" means; and this, we are told, is the "occupation of space." Now, the conception of "space," in the opinion of most psychologists, is ordinarily derived from our interpretation of visual sensations; and yet these may be altogether deceptive. When we look at a window from a short distance, we can not tell by the use of our eyes alone whether the space included by its frame is void, or is occupied by a perfectly transparent and colorless glass. A glass globe is held up in front of it, and we can not tell by looking at it whether it is empty, or is filled with pure water or some other transparent colorless liquid. And we can take no cognizance by our vision of the atmosphere which surrounds us, unless its transparence is interfered with by mist or fog. Clearly, then, our visual sense can not per se furnish us with a satisfactory definition of matter.[4]

Now that we have got rid of the fiction of "imponderables," we might fall back on a definition of matter—in use before that fiction was invented—as that which possesses "ponderosity" or weight. But what is weight? The downward tendency, it may be replied, in virtue of which all unsupported bodies fall to the earth. But what is this "tendency"? We might see any number of bodies falling to the ground, and might frame a correct law of their motion, without having the remotest conception of their possessing that downward pressure which we at once recognize when we take a lump of lead or iron into our hands; and it is obviously on our cognition of this pressure, that our idea of weight or ponderosity is based. Now, the instrumentality through which we take cognizance of it seems to me to be threefold: In the first place, we have the sense of simple pressure on the tactile surface—as when, the hand passively resting on a table, a weight is laid upon it. Secondly, we recognize it by the sense of tension which we experience when a weight is attached to a pendent limb, and which we refer to the muscles and ligaments which are thus put on the stretch; or when, the hand resting on the top of a cylinder of glass placed over an air-pump, the air is exhausted from beneath, so as to make us feel the downward "pressure of the atmosphere." In these two cases, the mind is the passive recipient of the sensory impressions. But, thirdly, when we determinately lift a weight or hold it suspended by our hands, we experience, in addition to the sense of pressure and the sense of tension, a sense of effort, which we recognize as an immediate revelation of consciousness, not referable to any physical impression, but of the same kind as that which we experience in a purely mental act, such as the fixation of the attention. And a little consideration will, I think, make it clear that it is on this "sense of effort" in resisting downward pressure that our cognition of weight is essentially based.

For, in the first place, the continuance of a moderate pressure on the cutaneous surface, like other sensory impressions that become habitual, soon ceases to affect us sensorially; for we cognosce rather the changes in the states of our sense-organs than the states themselves. Or, again, we may suffer under a temporary or permanent paralysis of the cutaneous sense, that may prevent our feeling the contact of the body we are lifting or supporting; and yet, recognizing its downward pressure in other ways, we can put our muscles into action to antagonize it. But, secondly, this paralysis may extend to the muscular sense, so that the feeling of muscular tension is wanting as well as that of contact-pressure; and yet none the less can a weight be lifted or sustained by a conscious effort, provided that the deficiency of the guiding sensations ordinarily derived from the muscle itself is supplied by the sight. A woman whose arm is sensorially but not motorially paralyzed can hold up her child as long as she looks at it, and a man affected with the like paralysis of his legs can stand and walk while looking at his feet. But, thirdly, since the mental sense of effort is experienced in every determinate exercise of our muscular power, and is, as all experience teaches, a necessary condition of that exercise; since, again, it is proportioned to the exertion we put forth, and continues as long as that exertion is sustained—it is in this, and not in the cutaneous or muscular impressions which are (so to speak) accidental, that (as it seems to me) we find the real basis of our cognition of the "ponderosity" of matter.

But "ponderosity" can not be considered an essential property of matter, being merely the "accident" of the earth's attraction for bodies lying within its range. This attraction varies with the distance of a body from the center of the earth; and a body occupying the common center of gravity of the earth and sun would be equally drawn toward both, and would consequently have no "weight." We must, therefore, seek a satisfactory definition of matter elsewhere; and we find the clew to it in the consideration that the sense of effort we experience in antagonizing the downward pressure of a body is but a particular case of our more general cognition of resistance. When we project our hand against a hard and fixed solid body, our consciousness of its resistance to our pressure is exactly that which we experience when we try to raise a weight that we have not strength to lift; while, if that solid be either yielding in its parts or movable as a whole, we measure its resistance, as in lifting a weight, by our sense of the effort necessary to overcome it. When we move our hand through a liquid, we are conscious of a resistance to its motion, which is greater or less according to the "viscosity" of the liquid. And, when we move our open hand through air at rest, we are still conscious of a resistance, our sense of it being augmented by an extension of the surface moved, as in the act of fanning; while, if the air is in motion, we feel its pressure on the sail of a boat by the "pull" of the sheet we hold in our hand, or on the sails of a windmill by the rotation it imparts, the force of which we can estimate by the effort we must put forth to resist it. Attenuate any kind of air or gas as we may, its resistance can still be made apparent by the like communication of its own motion to solid bodies. Thus, in Mr. Crookes's wonderful radiometer, a set of vanes, poised on a pivot within a globe of glass exhausted to a millionth of its ordinary gaseous contents, is whirled round by the movement excited in the molecules of that residual millionth, either by the heat of the radiant beam falling on the surface of the globe or by the passage of an electric current across its interior; and the mechanical force required to impart that motion can be measured with precision, by bringing it into comparison with some other force (as that of gravity) of which we can take immediate cognizance. And thus, as Herbert Spencer remarks, by the decomposition of our knowledge of any form of matter into simpler and simpler components, we must come at last to the simplest, to the ultimate material, to the substratum; and this we find in the impression of resistance we receive through what we may call our "force-sense."[5]

Such being the teachings alike of general and of scientific experience, I can not but feel surprised that any persons claiming the title of philosophers should affirm that we know nothing except matter and motion, and that force is a creation of our own imagination. One might suppose such persons to be either destitute of the "force-sense," or to have based their philosophical system upon the movements of the heavenly bodies which they can only see, instead of upon those mundane phenomena in the cognition of which they can bring their hands to the assistance of their eyes. How essential this assistance is to the formation of correct conceptions of the solid forms and relative positions of the objects around us is known to every one who has studied the physiology of the senses. Should we not think it absurd on the part of any one who possesses in the use of his hands the means of detecting the error of his visual perceptions, if he were to base a superstructure of reasoning—still more to found a whole system of philosophy—upon the latter alone? Yet such appears to me to be the position of those who deny our direct cognition of force.

Let us suppose (if possible) a man who had enjoyed the full use of his eyes, but whose limbs had been completely paralyzed from infancy, looking on at a game of billiards. He would see a succession of motions connected by regular sequence—the motion of the arm of the player, the stroke of the cue, the roll of the ball, its contact with another ball, the movement of the second ball, the change of direction or the entire stop of the first, the rebound of balls from the cushion in altered directions, and so on. And he might frame a statement in "terms of motion" of all that passes before his eyes, thinking this all he can know. But suppose the limbs of such a man to be suddenly endowed with the ordinary powers of sensation and movement; let him take the cue into his hands and himself strike the ball; let him hold his hand on the-table so that the rolling ball shall strike it and make him feel its impact; let him hold the second ball and feel the shock imparted to it by the stroke of the first. Can any one deny that he would thus acquire a dynamical conception linking together the whole succession of phenomena, which he was previously quite incapable of forming; that this dynamical conception is quite as directly based upon the experience derived through his "force-sense" as his kinetic expression was upon that derived through his visual sense; and that this cognition of the force producing the motions is, therefore, fully as much entitled to be introduced into a logical doctrine of causation as the visual cognition of the motions themselves? If it be replied that we have no proof that the movement of the ball we strike is produced by the force which we consciously exert in striking it, I simply rejoin that we have as much proof of it as we have of anything which rests upon universal experience, and which we can verify experimentally as often as we choose to try—quite as much as we have of the existence of anything whatever that is external to ourselves.

Let us take, again, the simple case of magnetic attraction. A man who knows nothing of magnetism sees a piece of iron, brought within a certain distance of what looks looks like a horseshoe bar of the same metal, suddenly jump toward its approximated ends; and might, as before, correctly express the fact in "terms of motion." But let him take the piece of iron in his hands, so as to feel the "pull" upon it when brought sufficiently near the magnet, and he then becomes conscious, through his force-sense, of a power of which he was before utterly ignorant.

Thus, as it seems to me, an analysis of those physical experiences on which all our cognitions of the physical universe around us are really based irresistibly lands us in the conclusion that, as Herbert Spencer expresses it, "All the sensations through which the external world is known to us are explicable by us only as resulting from certain forms of force"; the direct derivation of our conception of force from our own experience of muscular tension (or, as I should myself say, from our own sense of effort) being "a fact which no metaphysical quibbling can set aside." In the words of the able American writer I have already quoted, "The conception of force is one of those universal ideas which belong of necessity to the intellectual furniture of every human mind." By no one has the principle for which I am contending, been more clearly and more authoritatively expressed than by Sir John Herschel, a philosopher who united to his wonderful grasp of Nature-phenomena a profound insight into the action of the mind of man in the interpretation of them:

Whatever attempts have been made by metaphysical writers to reason away the connection of cause and effect, and fritter it down into the unsatisfactory relation of habitual [unconditional] sequence, it is certain that the conception of some more real and intimate connection is quite as strongly impressed upon the human mind as that of the existence of an external world, the vindication of whose reality has, strange to say, been regarded as an achievement of no common merit in the annals of this branch of philosophy. It is our own immediate consciousness of effort, when we exert force to put matter in motion or to oppose and neutralize force, which gives us this internal conviction of power and causation, so far 'as it relates to the material world.—(Treatise on "Astronomy" in Lardner's "Cyclopædia," p. 232.)

Man's position as the "interpreter of Nature" may be not inaptly likened (as it seems to me) to that of an intelligent observer of the working of a cotton-factory, with whose mechanical arrangements he is entirely unacquainted, and of whose moving power he knows nothing whatever. He is taken into a vast apartment,[6] in which he is at first utterly bewildered by the number and variety of the movements going on around him; but, by directing his attention to the several machines, seriatim, he is able to arrive at a classification of them, according to the kind of work which it does. Thus he finds one set carding the cotton-wool supplied to it, so that its confused tangle gives place to a parallel laying of the fibers. He would see another taking up the bundles of carded wool, and drawing them out (after repeated doublings to secure uniformity) into a long, soft cord. This cord he would then trace into the roving-machine, which, by a continuation of the drawing process, further reduces its thickness, at the same time giving it a slight twist to increase its tenacity, so that it admits of being-then wound upon bobbins. Thence he would trace the cord into the spinning-machine, which at the same time stretches and twists the cord, producing from it a yarn whose fineness might vary considerably in different machines. Finally, he would see the spun yarn carried, some as weft and some as woof, into the power-loom, from which it emerges as woven cloth—the final resultant of the whole series of operations.

Concentrating now his attention upon any one of these machines, he studies its wheels, levers, and other moving parts, and tries to comprehend their several actions and the bearing of these upon each other. By long and scrutinizing observation he masters the whole series of sequences, and traces the distribution of motion from a single large axis, through the hundreds (it may be) of separate pieces of the machine directly or indirectly connected with it; and he might thus frame a description of the working of the machine, which might be perfectly correct so far as it goes, and which yet would be defective in one most essential particular—the statement of the force or power by which it is moved. For, so far as mere visual observation could teach him, the machine might be self-moving; and he might thus attribute to each kind an inherent power of carding, roving, drawing, spinning, or weaving, as the case might be.

Carrying his observations further, and noticing that one or another of these machines comes to a standstill, but resumes its motion after an interval, he may include this occasional suspension also in his general expression; but, perplexed by the want of any regularity in its intervals, he will seek some further explanation. Continuing his patient watch, he will see that the stoppage of the machine follows the pulling of a handle by the man in attendance upon it, and that, when the handle is pulled the other way, the machine goes on again; and thus he will be led to introduce a certain position of this handle as one of the antecedent conditions of the machine's action. Still pursuing-his inquiries, he finds out that the axes of the several machines are all in mechanical relation with one great longitudinal shaft, being connected with it either by continuous bands passing around pulleys, or by trains of wheelwork; and at last he discovers the important fact that the movement of the handle which stops the machine breaks the continuity of that relation, shifting a strap from a "fast" to a "loose" pulley, or throwing the wheel work "out of gear"; while the converse movement, which restores that continuity, is followed by the renewed action of the machine, which goes on until the continuity is again broken. Thus he will be led to regard its maintenance as essential to the working of the machine; but nothing that he has yet learned explains to him why it is essential. He has only got at the material collocation which his educated vision enables him to recognize; and, for anything he knows to the contrary, the change in that collocation may be in itself adequate to determine the result.

But let him lay hold of the band which stretches between the main shaft and the axis of one machine, or attempt to stay with his hand the rotation of the train of wheels which connects it with another—he then at once becomes conscious, through his "force-sense," of the power which the band or the wheelwork is the instrument of conveying; and as he finds that the "pull" upon his hand is just the same whether the machine is in motion or not, provided that the band or wheel remains in mechanical connection with the main shaft, he comes to the conviction that the source of the power is in the shaft, and that, so far from any one of the machines having an inherent power of movement, its motion entirely depends upon the force supplied to it from the shaft. And when, under the guidance of this conception, he again examines the working of the several kinds of machine, he finds that, while the power is the same for all, the diversity in their respective products is traceable to the diversity in their construction—that is, to the material collocations through which the one moving force exerts itself in action.

But, having thus acquired the notion of moving power, and having satisfied himself of the derivation of the force that gives motion to each of the entire aggregate of machines, from one main shaft, our inquirer finds himself again posed. Has this shaft itself an inherent power of motion; or does it derive that power from any ulterior source? He sees the shaft apparently terminate in the two end-walls of the building; and, finding no evidence of its connection with anything else, he may feel himself drawn toward the conclusion that it moves of itself—that is, by the "potency" of its own material constitution. But, before adopting this rationale, he sees all the machines stop at once, and finds that the shaft also has ceased to revolve. Here is a new and startling phenomenon. After pondering on it for an hour, and carefully looking out for an explanation, he sees the shaft and its connected machines resume their motion, and yet is certain that no agency visible to him has had any concern in that renewal. By continued watching, he finds this suspension and renewal to be periodical, so that he can frame a law that shall express them in terms of time. Thus he might give a complete phenomenal account of the action of the shaft which should be perfectly consistent with the assumption of its "inherent potency," and which might be sufficiently satisfactory to his mind to justify him in believing that there is no more to be learned about it. But, not wishing to leave anything uninvestigated, he goes round to the other side of the wall. There he finds that one end of the shaft comes through it, and is in mechanical connection with either a steam-engine or a water-wheel; and, by watching what occurs when its motion is checked and renewed, he sees that the engineer shuts off, or turns on, either the steam generated in the boiler of the steam engine, or the descending water whose motion drives the wheel.

I shall not weary the patience of such readers as may have followed me thus far, by tracing out in like detail the further steps of the inquiry, but shall land them in the final conclusion now accepted by every man of science—that the power exerted in both these cases is drawn from solar radiation: the fall of the water which gives motion to the water-wheel being merely the return of that which has been pumped up as vapor by the sun's heat; while the combustion of coal from which steam-power is derived reproduces, as active force, or "energy," the sunshine that exerted itself during the Carboniferous period in dissociating carbonic acid and water into the hydrocarbons of coal and the oxygen of the atmosphere, whose recombination gives forth heat and light. And, if we look still further back for the source of the sun's radiant energy, we should find it, perhaps, in the progressive consolidation of the primeval "fire-mist"—nebular matter.

But whence nebular matter? And whence the force which draws its particles together, and which manifests itself as light and heat during their consolidation? Here we come to a wall, to the other side of which we seem at present to have no access.

But is there no other side? Does not the whole course of the preceding inquiry show the unsatisfaction (if I may revive an obsolete word) of resting in any inherent "potency" of matter as the ultima ratio of the existing cosmos? If we think the man foolish who supposes the main shaft of a cotton-mill to turn of itself merely because he sees it apparently end in a wall which conceals from him the source of its motive power, are we not really chargeable with the like folly if we attribute self-motion to the ultimate molecules of matter, merely because the power that moves them is hid from our sight? The mere physicist may see no possible way further. But there is a philosophy which has fully as true and as broad a basis in man's psychical experience as can be claimed for the fabric of physical science; and, in the admirable words of the great master I have already quoted (Sir John Herschel, in his "Familiar Lectures on Scientific Subjects," p. 460), I shall sum up an argument which this paper is intended rather to illustrate and enforce by an appeal to the familiar facts of consciousness than to present in strict logical form:

In the mental sense of effort, clear to the apprehension of every one who has ever performed a voluntary act, which is present at the instant when the determination to do a thing is carried out into the act of doing it, we have a consciousness of immediate and personal causation which can not be disputed or ignored. And, when we see the same kind of act performed by another, we never hesitate in assuming for him that consciousness which we recognize in ourselves; and in this case we can verify our conclusion by oral communication. ... In the only case in which we are admitted into any personal knowledge of the origin of force, we find it connected (possibly by intermediate links untraceable by our faculties, yet indisputably connected) with volition, and, by inevitable consequence, with motive, with intellect, and with all those attributes of mind in which personality consists.

As a physiologist, I most fully recognize the fact that the physical force exerted by the body of man is not generated de novo by his will, but is derived from the oxidation of the constituents of his food. But holding it as equally certain, because the fact is capable of verification by every one as often as he chooses to make the experiment, that, in the performance of every volitional movement, that physical force is put in action, directed, and controlled by the individual personality or ego, I deem it just as absurd and illogical to affirm that there is no place for a God in nature, originating, directing, and controlling its forces by his will, as it would be to assert that there is no place in man's body for his conscious mind.—Modern Review.

  1. "On the Mutual Relations of the Vital and Physical Forces," in "Philosophical Transactions," 1850.
  2. "System of Logic," eighth edition, vol. i., p. 406.
  3. "Natural Theology of the Doctrine of the Forces." By Professor Benjamin Martin, of the University of the City of New York.
  4. According to Professor Bain, the conception of space is essentially based on the sense of muscular tension which, according to him, we experience in the ordinary movements of our eyes. But I am satisfied that this is physiologically erroneous. These movements are ordinarily guided, as Professor Alison long ago contended, and as Professor Helmholtz and I myself have since experimentally proved, by the visual, not by the muscular sense; and it is only when we put the muscles to an unusual strain—as when our visual axes converge on an object brought nearer and nearer to the eyes, or when we entirely exclude light from the retina—that we experience any sense of tension in their muscles.
  5. Herbert Spencer considers the congnitionof resistance to be essentially derived from the sense of muscular tension. I have already expressed my reason for now dissenting from this view, which I myself formerly held.
  6. In one of the flax-spinning mills belonging to the Marshalls, of Leeds, the whole of the work is done on one floor, covering, I believe, two acres of ground, instead of in the usual building of several stories.