Front Page Titles (by Subject) CHAPTER XV.: THE PERCEPTION OF MOTION. - The Principles of Psychology
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CHAPTER XV.: THE PERCEPTION OF MOTION. - Herbert Spencer, The Principles of Psychology 
The Principles of Psychology (London: Longman, Brown, Green and Longmans, 1855).
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THE PERCEPTION OF MOTION.
§ 70. Our ideas of Motion, Time, and Space, are so intimately connected, that it is extremely difficult to disentangle them. On the one hand, preceding chapters have shown that Space and Time are knowable only through Motion: on the other hand, it is by some contended, with great apparent truth, that Motion is unknowable except as in Space and Time; and that, therefore, notions of Space and Time must pre-exist. Taking which two positions together, there would really seem no course left but to adopt the Kantian hypothesis; and conclude that Time and Space are forms of sensibility, that are disclosed to consciousness in the act by which Motion is perceived. A closer consideration, however, will show that there is an alternative.
For though Motion, as known by the developed mind, cannot be conceived without accompanying conceptions of Space and Time; it does not therefore follow that Motion, as known by the undeveloped mind, cannot be conceived without such accompaniments. It does not follow that because the connection between the ideas is, in adult life, indissoluble, it was always so. The whole confusion has arisen from the totally unwarrantable assumption, that certain impressions received through the senses, were originally understood in a way just like that in which they are understood after the accumulation of an infinity of experiences—an assumption at variance with the established facts of Psychology. Do we not know that the daily rising and setting of the sun, are thought of in completely different ways by the clown and by the astronomer? Do we not know that the adult and the juvenile differ widely in the conceptions suggested to them by the action of a lever, a pulley, or a screw? Do we not know that the form of a house is comprehended by the child, after a manner in which the infant cannot comprehend it? Moreover, is it not admitted that much of our acquired knowledge becomes so consolidated as to disable us from dissociating its elements in our minds—that on grasping an apple we cannot, without great difficulty, so confine our consciousness to the sensations of touch, as to avoid thinking of the apple as spherical—that we find it utterly impossible, when looking at a neighbouring object, to shut out all thought of the distance, and attend only to the visual sensations? And when we unite these two general facts—first, that by the putting together of experiences the mind acquires conceptions quite different from those it originally had; and, second, that experiences which have been from the beginning invariably connected, and perpetually connected, become fused into conceptions that are undecomposable by any subjective contemplation of them—does it not become manifest, both that the adult's idea of Motion is entirely distinct in nature from the infant's idea of Motion, and that it has become impossible for the adult to think of Motion as the infant thought of it? The candid inquirer cannot doubt it. And not doubting it, he will see the vice of the assumption that what are necessities of thought to us, are therefore necessities of thought in the abstract. He will see that the phenomena must be dealt with, not by subjective analysis, but must be analyzed objectively—must be considered, not as they present themselves to our consciousness, but as they would present themselves to a consciousness unoccupied by foregone conclusions.
“But how,” it may be asked, “is it possible for us thus to deal with the phenomena? How can we legitimately speak of Motion as known in some form different from that in which we know it? How can we treat of a conception which we cannot ourselves have?” Very readily. For though in our adult consciousness of Motion, the ideas of Space and Time are inextricably involved; yet there is another element in that consciousness which we can very clearly perceive would remain, were the ideas of Space and Time absent. Though it is perfectly true that on moving my arm, even when in the dark, I cannot become conscious of the motion without being simultaneously conscious of a space traversed and a time occupied in traversing it; yet I find that the muscular sensations accompanying the motion, are altogether distinct in nature from the ideas of Space and Time associated with them. I find no difficulty in so far isolating these sensations in thought, as to perceive that the consciousness of them would remain were my ideas of Space and Time abolished. And I find no difficulty in conceiving that Motion is thinkable by the infant as consisting of these sensations, while yet the notions of Space and Time are undeveloped. Seeing then that Space and Time are knowable only through Motion; and seeing that the primitive consciousness of Motion may readily be conceived to have contained but one of the elements ultimately included in it; we are warranted in the inquiry whether, out of such a primitive consciousness of Motion, the consciousness we have of it may be evolved.
§ 71. To open this inquiry systematically, let us first look at the several data furnished by preceding chapters.
We saw that our conception of Space is a conception of the relativity of coexistent positions; that the germinal element of the conception is the relation between two coexistent positions; that every relation between two coexistent positions is resolvable into a relation of coexistent positions between the subject and an object touched; that this relation of coexistent positions between subject and object, is equivalent to the relation of coexistent positions between two parts of the body; and that thus the question—How do we come by our cognition of Space? is reducible to the question—How do we discover the relation of coexistent positions between two sentient points on our surface?
Our conception of Time we saw to be that of relativity of sequent positions—relativity of position in the series of the states of consciousness. We saw that the germinal element out of which this conception is developed, is a relation of position between two states of consciousness; and that every relation of position between two states of consciousness is known by the number of remembered intervening states.
Respecting Motion, we know that as, through it only are changes in consciousness originally produced, through it only can relations of sequent positions among states of consciousness be disclosed; and that for the same reason, through it only can be disclosed the relations of coexistent positions. At the same time we know that whether Motion is or is not originally cognizable in any other way, it is from the beginning cognizable through the changes of consciousness it produces. If it be subjective motion, as that of a limb, it is present to the mind as a continuous but varying series of sensations of muscular tension. If it be objective motion, as that of something traversing the surface of the body, or as that of something passing before the eyes, it is still present to the mind as a continuous series of sensations: in the one case the tactual sensations that result from touching a succession of points on the skin; in the other case the visual sensations that result from exciting a succession of points on the retina. And if the motion be both subjective and objective, as when one part of the body is drawn over another part, or when a limb is extended within view of the eyes, then it is present to the mind as a double series of sensations: in the one case, as a series of muscular sensations joined with a simultaneous series of tactual sensations; in the other case, as a series of muscular sensations joined with a simultaneous series of visual sensations. Finally, when the hand is moved over the body within view of the eyes, motion is present to the mind as a triple series of sensations—muscular, tactual, visual—occurring simultaneously.
Omitting for the present all consideration of the visual phenomena, let us now turn our attention to the question in which centres the whole controversy respecting the genesis of our ideas of Motion, Space, and Time: the question namely—How do we become cognizant of the relative positions of two points on the surface of the body? Such two points considered as coexistent, involve the germinal idea of Space. Such two points disclosed to consciousness by two successive tactual sensations proceeding from them, involve the germinal idea of Time. And the series of muscular sensations by which, when self-produced, these two tactual sensations are separated, involve the germinal idea of Motion. The questions to be considered then, are—In what order do these germinal ideas arise? and—How are they developed?
Already, in treating of visible extension (§ 58), and the visual perception of space (§ 62), and in showing how serial states of consciousness are consolidated into simultaneous states which become their equivalents in thought, the way has been prepared for answering these questions. The process of analysis partially applied to retinal impressions, has now to be applied, after a more complete manner, to impressions on the body at large. To this end, taking for our subject a newly-born infant, let us call the two points on its body between which a relation is to be established, A and Z. Let us assume these points to be anywhere within reach of the hands—say upon the cheek. By the hypothesis, nothing is at present known of these points; either as coexisting in Space, as giving successive sensations in Time, or as being brought into relation by Motion. If now, the infant moves its arm in such a way as to touch nothing, there is a certain vague reaction upon its consciousness—a sensation of muscular tension. This sensation has the peculiarity of being indefinite in its commencement; indefinite in its termination; and indefinite in all its intermediate changes. Its strength is proportionate to the degree of muscular contraction. Whence it follows that as the limb starts from a state of rest, in which there is no contraction; and as it can reach a position requiring extreme contraction only by passing through positions requiring intermediate degrees of contraction; and as the degree of contraction must therefore form a series ascending by infinitesimal increments from zero; the sensations of tension must also form such a series. And the like must be the case with all subsequent movements and their accompanying sensations; seeing that, be it at rest or in action, a muscle cannot pass from any one state to any other without going through all the intermediate states. Thus, then, the infant, on moving its arm backwards and forwards without touching anything, is brought to what we may distinguish as a nascent consciousness—a consciousness not definitely divisible into states; but a consciousness the variations of which pass insensibly into each other, like undulations of greater or less magnitude. And while the states of consciousness are thus incipient—thus indistinctly separated, there can be no clear comparison of them; no classing of them; no thought, properly so called; and consequently, no ideas of Motion, Time, or Space, as we understand them. Suppose, now, that the hand touches something. A sudden change in consciousness is produced—a change that is incisive in its commencement, and, when the hand is removed, equally incisive in its termination. In the midst of the continuous feeling of muscular tension, vaguely rising and falling in intensity, there all at once occurs a distinct feeling of another kind. This feeling, beginning and ending abruptly, constitutes a definite state of consciousness; and becomes, as it were, a mark in consciousness. By similar experiences other such marks are produced; and in proportion as they are multiplied, there arises a possibility of comparing them, both in respect to their degrees and their relative psitions: while at the same time, the feelings of muscular tension being, as it were, divided out into lengths by these superposed marks, become similarly comparable; and so there are acquired materials for a simple order of thought. Observe, also, that while these tactual sensations may, when several things are touched in succession, produce successive marks in consciousness, separated by intervening muscular sensations, they may also become continually coexistent with these muscular sensations; as when the finger is drawn along a surface. And observe further, that when the surface over which the finger is drawn is not a foreign body, but some part of the subject's body, these muscular sensations, and the continuous tactual sensation joined with them, are accompanied by a series of tactual sensations proceeding from that part of the skin over which the finger is drawn. Thus, then, when the infant moves its finger along the surface of its body from A to Z, there are simultaneously impressed upon consciousness three sets of sensations—the varying series of sensations proceeding from the muscles in action; the series of tactual sensations proceeding from the points of the skin successively touched between A and Z; and the continuous sensation of touch from the finger-end. Now it might be argued that some progress is made towards the idea of space, in the simultaneous reception of these sensations—in the contemplation of them as coexistent: seeing that the notion of coexistence and the notion of space have a common root; or in other words—seeing that to be conscious of a duality or multiplicity of sensations, is the first step towards being conscious of that duality or multiplicity of points in space which they imply. It might also be argued that as, when the finger is moved back from Z to A, these serial sensations are experienced in a reverse order, there is thus achieved a further step in the genesis of the idea: seeing that coexistent things are alone capable of impressing consciousness in any order with equal vividness. But passing over these points, let us go on to notice, that as subsequent motions of the finger over the surface from A to Z, always result in the like simultaneous sets of sensations, these, in course of time, become indissolubly associated. Though the series of tactual sensations, A to Z, being producible by a foreign body moving over the same surface, can be dissociated from the others; and though, if the cheek be withdrawn by a movement of the head, the same motion of the hand, with its accompanying muscular sensations, may occur without any sensation of touch; yet, when these two series are linked by the tactual sensation proceeding from the finger-end, they necessarily proceed together; and become inseparably connected in thought. Whence, it obviously results that the series of tactual sensations A to Z, and the series of muscular sensations which invariably accompanies it when self-produced, serve as mutual equivalents; and being two sides of the same experience, suggest each other in consciousness. Due attention having been paid to this fact, let us go on to consider what must happen when something touches, at the same moment, the entire surface between A and Z. This surface is supplied by a series of independent nerve-fibres, each of which at its peripheral termination becomes fused into, or continuous with, the surrounding tissue; each of which is affected by impressions falling within a specific area of the skin; and each of which produces a separate state of consciousness. When the finger is drawn along this surface, these nerve-fibres A, B, C, D,…Z, are excited in succession; that is—produce successive states of consciousness. And when something covers, at the same moment, the whole surface between A and Z, they are excited simultaneously; and produce what tends to become a single state of consciousness. Already I have endeavoured to show in a parallel case (§ 58), how, when impressions first known as having sequent positions in consciousness are afterwards simultaneously presented to consciousness, the sequent positions are transformed into coexistent positions, which, when consolidated by frequent presentation, are used in thought as equivalent to the sequent positions: and it is needless here to repeat the explanation. What it now concerns us to notice is this:—that as the series of tactual impressions A to Z, known as having sequent positions in consciousness, are, on the one hand, found to be equivalent to the accompanying series of muscular impressions; and on the other hand, to the simultaneous tactual impressions A to Z, which, as presented together are necessarily presented in coexistent positions; it follows that these two last are found to be the equivalents of each other. A series of muscular sensations becomes known as equivalent to a series of coexistent positions; and being habitually joined with it, becomes at last unthinkable without it. Thus, the relation of coexistent positions between the points A and Z (and by implication all intermediate points), is necessarily disclosed by a comparison of experiences: the ideas of Space, Time, and Motion, are evolved together. When the successive states of consciousness A to Z, are thought of as having relative positions, the notion of Time becomes nascent. When these states of consciousness, instead of occurring serially, occur simultaneously, their relative positions, which were before sequent, necessarily become coexistent; and there arises a nascent consciousness of Space. And when these two relations of coexistent and sequent positions are both presented to consciousness along with a series of sensations of muscular tension, a nascent idea of Motion results.
The development of these nascent ideas, arising as it does from a still further accumulation and comparison of experiences, will be readily understood. What has been above described as taking place with respect to one relation of coexistent positions upon the surface of the skin—or rather, one linear series of such coexistent positions, is, during the same period, taking place, with respect to endless other such linear series, in all directions over the body. The like equivalence between a series of coexistent impressions of touch, a series of successive impressions of touch, and series of successive muscular impressions, is being established between every pair of points that can readily be brought into relation by movement of the hands. Let us glance at the chief consequences that must ultimately arise from this organization of experiences.
Not only must there gradually be established a connection in thought between each particular muscular series, and the particular tactual series, both successive and simultaneous, with which it is associated; and not only must there, by implication, arise a knowledge of the special muscular adjustments required to touch each special part; but, by the same experiences, there must be established an indissoluble connection between muscular series in general and series of sequent and coexistent positions in general: seeing that this connection is repeated in every one of the particular experiences. And when we consider the infinite repetition of these experiences, we shall have no difficulty in understanding how their components become so consolidated, that even when the hand is moved through empty space, it is impossible to become conscious of the muscular sensations, without becoming conscious of the sequent and coexistent positions—the Time and Space, in which it has moved.
Observe again, that as, by this continuous exploration of the surface of the body, each point is put in relation not only with points in some directions around it, but with points in all directions—becomes, as it were, a centre from which radiate lines of points known first in their serial positions before consciousness, and afterwards in their coexistent positions—it follows, that when an object of some size, as the hand, is placed upon the skin, the impressions from all parts of the area covered being simultaneously presented to consciousness, are placed in coexistent positions before consciousness: whence results an idea of the superficial extension of that part of the body. The idea of this extension is really nothing more than a simultaneous presentation of all the impressions proceeding from the various points it includes, which have previously had their several relative positions measured by means of the series of impressions separating them. Any one who hesitates respecting this conclusion, will, I think, adopt it, on critically considering the perception he has when placing his open hand against his cheek—on observing that the perception is by no means single, but is made up of many elements which he cannot think of all together—on observing that there is always one particular part of the whole surface touched, of which he is more distinctly conscious than of any other—and on observing that to become distinctly conscious of any other part, he has to traverse in thought the intervening parts; that is, he has to think of the relative positions of these parts by vaguely recalling the series of states of consciousness which a motion over the skin from one to the other would involve.
It is needless now to dwell upon that further development of these fundamental ideas which results when the visual experiences are united with the tactual and muscular ones. Being merely a further complication of the same process, it may readily be traced out by joining with the above explanations, those given when treating of visible extension and space. It will suffice here to say that, by serving clearly to establish in our minds the identity of subjective and objective motion, sight finally enables us more or less completely to dissociate Motion in the abstract, from those muscular sensations through which it is primarily known to us; and that by doing this, and by so reducing our idea of Motion to that of coexistent positions in Space occupied in successive positions in Time, it produces the apparently necessary connection between these three ideas.
§ 72. Thus then, we find that Motion, originally present to consciousness under a far simpler form than that in which we know it, serves by its union with tactual experiences to disclose Time and Space to us; and that, in the act of disclosing them, it itself becomes clothed with the ideas of them; and ultimately becomes inconceivable without these ideas.
It remains to add that the perception of Motion, as we know it, consists in the establishment in consciousness of a relation of simultaneity between two relations—a relation of coexistent positions in Space, and a relation of sequent positions in Time. In other words, the consciousness of Motion is produced by a simultaneous presentation of these relations—a united cognition of them. And it is scarcely needful to say that in the act of perception, these jointly-presented relations are severally assimilated to the like relations before known—that the perception of great velocity, for example, is possible only by simultaneously thinking of two coexistent positions as remote, and two sequent positions as near: which words remote and near, imply the classing of the two relations with previously experienced ones. And similarly with perceptions of the kind of motion, and the direction of motion.