Perception and Idea

If we cross the fingers, a single object beneath them appears to be two; and yet we do not say that there are two, for sight is more decisive than touch; but if touch were our only sense, our judgment would declare that the single object is two.—Aristotle

§ 24. The Problem in General.—The chapters on the mental elements—sensation, simple image, feeling—have made you acquainted with the results of psychological analysis; it was only occasionally that you were asked to analyse for yourself. Henceforth we shall be dealing with experiences that offer themselves for analysis; with experiences that, however simple they may at first sight appear, turn out on investigation to be complex. We shall thus be following the example of those men who, long centuries ago, tried to bring order into mental phenomena and to establish a science of mind. We have an enormous advantage; for they were working in the dark, and we are working in the light of their discoveries. Still, our procedure will be the same as theirs; and the change of work brings with it certain difficulties that you must realise at the outset and be ready to face. Well begun is half done.

First of all, then, your reading henceforth will be more difficult, because you will have to keep more things in mind. The analysis even of so comparatively simple a thing as a perception or idea cannot be performed in one breath. A knot in a rope may be beautifully simple, and yet you may spend a week in learning it! Secondly, the examples chosen by the author may not be just the right examples for you; even perceptions and ideas, again, differ a good deal in different minds; and an example that is illuminating to one reader may leave another quite blind. So you must look for your own examples in your own experience. Thirdly, you have now to wrestle with the problem of meaning (p. 26); for all perceptions and ideas, and all our still more complicated experiences, mean something; a perception is always the perception of a tree or a wedding or what not; and an idea too is always the idea of something, whether of the landing of Columbus or of the quarrels of the gods in Homer. You must get clear, then, about the psychology of meaning. Fourthly, these concrete experiences that you are to analyse have a long history; and in seeking their nervous correlates we shall be obliged, oftentimes, to go far back, even beyond the individual, to the development of the race. In doing this we do not change the problem of psychology (p. 18), but we enlarge our view of it; a mere reference to the organ of sense or the present condition of the nervous system is no longer enough.

All this means, in summary, that we are passing from the abstract to the concrete, from the meaningless to the meaningful, from the simple to the complex. We still keep to our scientific point of view, and we still employ our scientific method. The change is not in us, who are psychologising, but in our subject-matter; the plot begins to thicken; and this growing complexity of subject-matter naturally makes increasing demand upon our scientific resources.

§ 25. The Analysis of Perception and Idea.—Sensations and simple images can hardly occur, by themselves alone, in our everyday experience. The practised psychologist may be able to focalise a sensation, to make it so vivid that it stands out almost as it would under the experimental control of the laboratory; but his is an exceptional case. The units of our daily experience are rather such things as the sound of the piano in the next room, the sight of the tree budding just outside the window, the memory of last winter’s snow-piles, the forecast of to-night’s Pathetic Symphony; that is, they are perceptions and ideas. Notice that they come to us in the first place as units, as wholes; they show no lines of natural cleavage; they are unitary and self-contained. Yet they are not psychologically simple; if they were, we should never have lit upon sensations and simple images. All perceptions and ideas may be analysed.

A typical perception resolves, to begin with, into a number of sensations. The sound of the piano is, after all, the sound of certain compound tones, played together and in succession; and the sight of the tree is an arrangement of colours. The characteristic part of a perception, then, the part that we may conveniently call its core or nucleus, may thus be analysed into sensations. Only the core, however; for the sensations are supplemented, secondly, by various images. The sound comes to us as the sound of the piano, the instrument of that familiar look; and we may have an imaginal hint of the child playing, of the score, of its special difficulties, of all sorts of related things. The tree, too, is that tree, the familiar cherry that the caterpillars infest so badly, that grew so much last year, that will presently cut off the view across the street, that very likely will interfere with the beech. Remember that these are the author’s instances, and that you must replace them by your own! The point is that the complement of images is there; and you will notice that it is not stable; it may be full or scant, and it may lead the mind this way or that; but, whatever it be, it puts more into the perception than the sensory stimuli can account for; we perceive more than we hear or see.

Yes, and we perceive more than is furnished us by sensations and images. It is a fact (which you will better understand presently) that every perception is shaped and moulded by the action of nerve-forces which show themselves neither in sensation nor in image. The nervous system, whether by racial heritage or by individual habit, meets its impressions halfway, and throws them into certain customary forms. We take both the tree and the piano to be real things, and we take them to be things that occupy real space; we perceive them as objects of the outside world, and we perceive them as solid or space-filling. We do this because we have a natural and ingrained tendency to cast our perceptions into the forms of ‘thing’ and ‘space’; and this tendency of the nervous system does its work automatically; it has no correlate of sensation or image; but it is none the less effective, so to say, behind the sensations and the images, in determining the perception. You must just accept this statement now; it will become clearer later on.

A typical idea, in the same way, has a core or nucleus of images. Last winter’s snow may come to us in many different ways, because our equipment of images is very variable (pp. 75 f.); it will come to most, perhaps, as a visual picture, an uneven spread of white, with streaks of grey-brown on the peaks and along the valleys, honeycombed and broken from some partial thaw. To-night’s music will come, possibly, as the sound of the opening adagio measures, or of some theme from the allegro. Here again, however, the nucleus has its surroundings; other images cluster about it; we recall the day so-and-so got his feet wet, or the big fall of that December Thursday; we see our place in the concert-hall, or hope that this time the tympani will be in tune. Nor is the idea altogether a matter of images. We can hardly think of those opening measures without kinÆsthetic sensations from the throat, or from some muscular beat of the rhythm; we can hardly think of getting our feet wet, or of seating ourselves in the hall, without some actual movement that arouses sensation. Find your own instances, once more, and do not trust the author! You will find that the typical idea is thus in part sensation, just as the typical perception is in part image. Finally, the idea, too, is subject to the pressure of the directive nerve-forces; it takes the same customary forms as the perception. Columbus is thought of as a real person, acting in a real world of space and time; and Zeus as an imaginary person in an imaginary world; but there is no difference in the form of the ideas, and no difference of form between these ideas and the perception of the stranger who has just passed the window.

So we have the characteristic nucleus; the varying complement; and the brain-habit behind all. And if we can analyse the perception or idea, nucleus and surroundings both, into its mental elements; if we can say what nervous processes are correlated with these elementary mental processes; and if we can further establish the nature of the guiding and shaping nerve-forces; then our psychological account will be, in strictness, complete. Yet we shall have passed over something that, as we have ourselves admitted, is in everyday life most strikingly characteristic of these experiences; the fact, namely, that they mean; that our perception of the tree means the tree, is a perception of that tree, and our idea of snow means the snow, is an idea of that snow. What, then, from the psychological point of view, is this meaning?

§ 26. Meaning in Perception and Idea.—We learned in § 6 that mental processes are not intrinsically meaningful, that meaning is not a constituent part of their nature. We have seen, indeed, that the whole notion of meaning is really foreign to science. When we ask, then, what meaning is, from the psychological point of view, are we not asking an irrelevant and unscientific question?

Not necessarily. A science cannot free itself, offhand, from its own history; and, historically, psychology has been much concerned with meaning. Moreover, meaning is of very great practical importance; we communicate meanings, we apprehend meanings, we act upon meanings; and although science is not bound to treat only of what is practically important, yet it can hardly neglect a matter of great practical importance that comes its way. Our question, if we rephrase it a little, merely asks that a term, familiar to us in our daily life, be translated into the language of science; and if the translation out of common sense into science is to be made at all, psychology is the science in which the equivalent of meaning will be found. For these reasons we are justified in discussing the matter here; and the question at issue—let us be quite clear about it—is this: What mental processes, in perception and idea, are the scientific equivalent of what we know in everyday life as meaning? what processes carry the meaning?

The answer is that the processes which surround the nucleus carry the meaning. Psychologically regarded, meaning is always context; and the context is the fringe of related processes that gathers about the central group of sensations or images. Ordinarily, as our analysis has shown, the two come together; but they may be disjoined. When the word ‘house’ becomes meaningless with repetition (p. 26), it is because the bare sound grows more and more vivid and dominant; like the nestling cuckoo, it drives out its normal associates; and these associates, the carriers of its meaning, sink lower and lower into the obscurity of the background. So the meaning, almost literally, drops off, falls away. When one and the same experience has different meanings, it is because the context varies; we read, for instance, that so-and-so received a warm welcome, and we put directly opposite interpretations on the words, according as so-and-so was friend or enemy. When we mistake a meaning, it is because we supply a context of our own: what child, reading that “the quality of mercy is not strain’d,” has not thought of mercy being wrung out through a strainer, as the cook wrings the water out of cottage-cheese in a muslin bag? The context of images is obvious; the rain falls freely, like water poured through a sieve; but what is strained comes out grudgingly in drops. When one and the same meaning attaches to several experiences, it is because these different experiences are received into the same context, or into a context so nearly the same that for practical purposes the differences disappear; for example, the experiences may be named, that is, may be received into a context of verbal ideas; and verbal ideas tend to become stereotyped, as it were, into permanent groups. All the facts of § 6 are to be accounted for in this way, by the distinction of nucleus and context.

Originally, we must suppose, meaning was carried exclusively by kinÆsthetic and organic sensations. Think of the animal that we pictured on p. 101 as startled by some sudden stimulus and as facing the stimulus by way of a bodily attitude; the sensation is hemmed in, like a jewel in its setting, by the sensations of organic stir and motor posture; and these sensations give the meaning; they cry out ‘Danger!’; they are the psychological equivalent, the carriers, of that meaning; without them the sensation would be meaningless. Meaning is thus older than the free image; and kinÆsthesis is still, for many of us, the characteristic context, the common denominator of our meanings; we hinted at this rÔle on p. 47. None the less, the development of free images, the images of memory and imagination, changes the whole situation; kinÆsthesis now has many rivals; and it depends on our individual equipment of images, on our ‘type of mind,’ whether a meaning shall be carried by a quiver of the stomach or some muscular set, or whether it shall be carried by some complex of images. If we were to work out a great number of cases, we should probably find that any sensory or imaginal process whatsoever is able, in our adult human experience, to carry the meaning of any other.

There is yet a further stage: a stage in which meaning is carried not by any sort of sensation or image, but simply and solely by physiological processes, by some set or disposition of the brain. When the practised reader skims a number of pages in quick succession; when the musician renders a composition in the prescribed key; when an accomplished linguist shifts from one language to another as he turns to his right or left hand neighbour at a dinner table; in cases of this kind there need be no discoverable context; the stimuli press the button, and the brain, prepared by constant practice in the past, now does the rest. The experiences mean, positively enough; the ‘sense’ of the pages is grasped, as the eye hurries over the lines; the three flats on the staff set the player’s hand and eye for the key of E?; the question put in French is suitably answered in the same language; everything takes place as if there were a fringe of images that gave meaning to the bare perceptions; and yet imaginal fringe and kinÆsthetic setting may be conspicuous only by their absence. Of course, there has been context; one does not learn French and German, or transpose on the piano, by gift of a ready-made nervous system; even after years of work one may be a little uncertain of the German auxiliaries, or have a repugnance to four sharps. The point is, however, that an habitual and often-repeated context does, presently, lapse altogether; the nucleus is not always supplemented; the nervous system can now do, by a set or disposition that has no mental correlate, what it used to do by processes that had as accompaniment the sensations or images of the context.

It is plain, therefore, that perception and idea are not always so rich and complicated as we have described them; we spoke, for that very reason, of the ‘typical’ perception and idea. They range, according to their age and use, from the cluster of nuclear processes surrounded by a group of contextual associates, all under the guidance of a directive nerve-force, down to a mere rag and tag of sensory or imaginal process, wholly bare of associates, and dependent for its meaning upon some habitual nervous set.

§ 27. The Types of Perception.—Our perceptions are based upon three of the attributes or aspects of sensation: upon quality, upon duration, and upon extension (p. 66).

The quality of sensation has already been discussed. We may take, as instances of qualitative perception, the taste of coffee, the resistance of a jammed door, and the note of a musical instrument. The taste of coffee analyses into sensations of bitter, the real taste of the coffee-berry; of warmth; of pressure, the feel of the liquid in the mouth; and of a peculiar fragrance, the odour of coffee. Along with these goes a colour, the clear or clouded brown of the coffee in the cup, and various other contextual processes. The resistance analyses into the qualities of pressure from the skin; of strain from the tendons of the arm; and of pressure, or something akin to pressure, from the binding of the joints and the contraction of the muscles. There is probably some organic stir; there is the sight of the door; and there may be a further context. The musical note analyses into fundamental tone and overtones, and into the noise characteristic of the instrument; the thud of the piano, the scrape of the violin, the pluck of the harp. The supplement is perhaps visual; but here, as in the other cases, verbal ideas may enter into the context; we may think ‘Violin, of course,’ All our qualitative perceptions are of this kind; they come to us as meaningful wholes, and they may be analysed into a number of sensory qualities, run or fused or blended together, and set in various contexts of associated processes.

The attribute of duration has not yet been defined. It is the bare going on, going forward, keeping like itself, that may be observed in any and every sensation; you recognise it most easily, perhaps, if you listen to a tone, or attend to the kinÆsthetic complex as you slowly extend your arm from the elbow. It is the elementary time-factor in all our perceptions of time,—in the perceptions of period, of interval, of rate, of rhythm, and so on; though in some of these perceptions it is overlaid and obscured by other factors. Qualitative perceptions undergo relatively little change, just because they are qualitative perceptions; the best and easiest way to mean a quality is to be it; the best way to mean the coffee-taste is to be the coffee-taste; and so our perception of that taste remains practically the same all our life long. Time-perceptions, on the other hand,—and the same thing is true of space-perceptions,—change enormously; the nervous system finds all manner of short-cuts to the meaning of time; and these short-cuts have to be unpractised, to be practised out, if we are to observe the perception in its original form. Thus, to take the simplest case, a period of time may seem long because the kinÆsthetic strain of waiting becomes intense, or because a great number of perceptions and ideas occur during its course; the strain and the number of ideas have come to mean length of time, and the primary experience of duration, so to say, drops out of sight. If, therefore, we wish our observers in the laboratory to compare periods of time; if we wish to find out accurately what durations can be grasped by the attention and held in the memory; then we must break them of these time-habits, and must somehow train them to disregard strain and to discard imagery. We cannot often carry the unravelling of a perception to the very end, though we can go some distance behind the appearances of everyday life.

The attribute of extension is the bare character of patch or spread that inheres in all sensations from eye and skin, and possibly also in kinÆsthetic and organic sensations. No point of light or pressure is so fine that it is not areal. Extension is the elementary space-factor in all our perceptions of space. It enters most obviously into the perception of surface, as duration enters most obviously into that of period; but it is the basis also of our perceptions of form, size, distance, locality, direction. Like duration, it is often obscured and overlaid by other factors.

Here, however, you will raise an objection. Have we not said, on p. 115, that perception is shaped and moulded by nerve-forces that have no mental correlates? and did we not take as an example the casting of perceptions into the forms of ‘thing’ and ‘space’? How, then, can we now speak of perceptions of space?—Well, for one thing, there are various kinds of spatial perception; and it will not do to assume that they are all alike a matter of brain-habit, without mental correlate. Secondly, however, there is a difference between perceiving the piano or the tree as spatial, and turning our attention directly upon its spatial characters, its size or form, its distance or direction. In the latter case, we may rightly speak of a perception of space; we may so speak, even if the various kinds of spatial perception do turn out to be matters of brain-habit; and we must examine every kind for itself, precisely in order to determine how far it is sensory and imaginal, and how far it is a form impressed on sensations and images by the trend of the processes in the brain.

So the objection is answered. Coming back to the subject, we note that some of our more complex perceptions have a twofold basis: thus the perception of melody is at once qualitative and temporal, and the perception of movement is at once temporal and spatial. Nay more, the perception of a scene, a situation, an event, is threefold: qualitative, temporal and spatial; think of a scene of grand opera, or of an accident on the street. In general, the analysis of these complex perceptions follows from that of the simpler modes, though every one of them has its own psychological problem.

It may seem strange that we have not distinguished a group of perceptions based upon sensory intensity. The fact is, however, that while intensity enters into all sorts of perceptions (lemonade must not be too sour, the members of a rhythm must be variously accented, a distant sound is faint), it only rarely characterises a perception; and when it does, the perception thus characterised belongs to one or other of the groups already mentioned. We say ‘What a heavy child!’—but the perception of weight, like that of resistance, is itself qualitative. Or we say of a certain composer ‘He always overdoes the drums!’—but the drum-rhythm is itself a temporal perception. We cannot point, then, to a separate class of intensive perceptions.

§ 28. The Perception of Distance.—A complete psychology of perception would contain an analytical treatment, up to the limits of our present knowledge, of all the various perceptions, qualitative, temporal and spatial, as well as complex, that occur in experience. Such a treatment is here out of the question. We must pick and choose; and as a sample of perception at large we shall consider the perception of distance. We seem, quite immediately and directly, to see distances; we see that our friend is coming nearer, we see that he has passed the bridge, we see that he is entering the gate, we see when to shake hands with him. Yet there is no sensation of distance, and there is no specific stimulus to distance. What, then, really happens?

In the first place, there are plenty of visual cues to distance. We take familiar things to be far off if they look small, and near by if they look large; the size of the men and vehicles in the street makes us realise the height of the building we are gazing down from. We take things to be far off, again, if they are hazy and bluish, near by if they are clearly outlined and varied in colour; everyone knows or has read of the deceptive nearness of distant mountains in clear dry air. We notice the distribution of light and shade; a morning or evening landscape, a shaded face or sphere, looks deeper, more solid, more plastic, than the landscape at high noon or the outline drawing. We notice the course of boundary lines and the visibility of surfaces; that is nearer which cuts across the rest or blots part of it out; the telephone wire is thus nearer than the elm, and the elm is nearer than the house. We notice the number of objects that the eye must traverse to arrive at its goal; and the more numerous the objects, the farther off do we take the goal to be; the town looks near, we say, but there are all those fields, and the wood, and the churchyard, and half-a-dozen farmhouses to pass, and then the outlying houses; it must be a good two miles. We get various hints from movement; a crawling train or car is far away; and if we are looking at a near object and move the head to one side, distant objects move in the same direction, while if we are looking at a far object and move the head, near objects go in the opposite direction; and so on. All these things—linear perspective, aerial perspective, chiaroscuro, interposition, number, movement—are, however, secondary affairs; they represent short-cuts to the meaning of distance (p. 123); they do not lead us to the perception of distance itself. At the same time, we should bear in mind that these secondary processes were there, ready to take up the burden of meaning, all the while that the perception was forming.

Having thus cleared the ground, we naturally appeal to experiment; but unfortunately the first step that we take lands us in difficulties. It is found that, when all the cues above mentioned are ruled out, the estimation of distance is still possible; and many psychologists believe that it depends upon kinÆsthetic sensations set up in and about the eye. Each eyeball is slung in its orbit upon six muscles; and the contraction of these muscles is, naturally, greater for convergence of the eyes upon near objects than for their convergence upon far; so that the sensations of convergence seem fitted to play a part in the perception of distance. If only one eye is used, these sensations may be replaced by others, derived from the muscular system, within the eyeball, that adjusts or accommodates the lens for clear vision at different objective distances. The sensations of accommodation, though, in ordinary binocular vision they are entirely subordinate to the sensations of convergence, can nevertheless—within a lesser range of distances—play the same part in perception. Unfortunately, as was hinted just now, the results of these experiments are disputed; we shall come back to them, and to the possible rÔle of the kinÆsthetic sensations, later on.

Meantime, what is to be said of the eyes themselves, and of the impression that a solid object, a tridimensional stimulus, makes upon them? If you hold up a closed book, back towards you, in the middle line of the face, and if you observe it alternately with the right and left eye, you will find that the two views do not tally; the left eye sees the back and the cover to the left, the right eye sees the back and the cover to the right. If you now make outline drawings of the two views, mount them upon a suitable card, and look at them through a stereoscope,—which, as you know, combines them into a single view,—lo! you have before you a solid book, the back near you, and the edges away in space. It is as if the two eyes had reconciled their conflicting views, and the result were depth or solidity.

But is not this the very thing we were in search of? have we not at last got at the secret of visible depth? No; we are rather at the crucial point of our discussion. For this binocular picture, the image seen in the stereoscope, cannot be, of its own nature and in its own right, deep or solid, unless there is a depth-sensation; and that conclusion goes against everything that we know both of sensation and of the stimuli that arouse sensation. To avoid it, some psychologists call in the kinÆsthetic sensations from the muscles of the eye. Depth or distance, they say, is psychologically a blend or fusion of visual and kinÆsthetic sensations. Our binocular view of the book, its appearance to the two eyes, is in itself flat; but we run the eyes over it, and the muscular sensations thus blend with the visual. Nay more, even if we hold our eyes fixed, there is still a tendency to move them; and this tendency, now ingrained in our nervous system, is enough to realise the perception. Indeed, if experiment fails in every case to show the sensations of convergence and accommodation, that is just because the fusion is so long-established and so ingrained; we perceive distance, the fusion itself; we can hardly expect to recover the kinÆsthetic sensations that originally entered into it; the wonder rather is that they should ever appear, that experiment should be able to reveal them at all.

No one can say positively that this hypothesis is wrong; but it is difficult to believe that the blend of visual and kinÆsthetic sensations should yield a result so different from either,—namely, the perception of space. It seems safer to say that the binocular picture, the appearance of the book to the two eyes or the combined image of the stereoscope, carries the immediate meaning of depth or voluminousness. The picture is not itself deep or solid; but we cannot help perceiving it as deep and solid; and this pressure is laid upon us by what we have called racial heritage, an inherited disposition of the nervous system: the brain meets the impression halfway. The binocular picture thus becomes the core or nucleus of the perception; and the meaning of depth is carried by a nervous disposition that has no correlate in sensation or image. The kinÆsthetic sensations may then very well serve, as a secondary context, to give precision and accuracy to the perception, to develop the perception of crude voluminousness into the perception of definite distances. As to the nervous disposition, we can only say that it has been set up by the same biological causes that have made the organism a motor organism, one that moves freely in space; beyond that general statement we cannot go.

So far we have dealt with the space of sight; but there is also a space of touch; and we have next to ask whether the perception of distance can be couched in terms of touch alone. Our appeal lies to those who are born blind. Observations show that, in their case, the direct perception of solidity, of plasticity, is rare and fleeting; it arises, perhaps, when they clasp a child to their breast, or when they have been trained by long manipulation to distinguish objects of various shapes and sizes; it does not form a permanent item of their mental furniture. The blind behave as if they perceived distance; they avoid obstacles,—near obstacles by the pressure or temperature of the air reflected back upon their face, and remote obstacles by sounds; they can be taught geometry, and they measure objective distances by pacing; but the meaning of distance seems always to remain abstract, very much as the meaning of light and colour must remain abstract; there is no realising perception of distance. The brain mechanism which is ready to act at once at the behest of sight thus seems to be lacking where touch alone is present; even the perception of crude volume, of depth, has to be built up afresh by the individual. The blind live mainly in a world of sounds; touch is employed, as a rule, only for special and limited purposes, such as dressing, reading, handicraft; and their world is therefore not pervasively spatial, like the world of the seeing.

Go back now, for a moment, to the objection raised on p. 124! We have, as a matter of fact, been led to the belief that the meaning of depth is carried, in the last resort, by a brain-habit. But how differently does this sentence read before and after the discussion! You have learned something of the difficulties of the study of perception; you see why it is necessary to look at perception historically, developmentally; you have been taken behind the obvious visual cues to the perception itself; you have seen how the kinÆsthetic sensations and the binocular picture may be made the subject of experiment. Even the bare outline that the narrow compass of the present book allows should convince you that the objection was duly answered.

§ 29. The Problem in Detail.—Every one of our familiar perceptions might, now, be treated in this same fashion, and in indefinitely greater detail. We should start out with our pattern of sensory nucleus, imaginal context, and brain-habit; and we should push our analysis back and back, in the effort to reach the primary and ultimate form of the perception we were discussing. The quest is fascinating; for these are old, old bits of the mental life; to trace them home would be to go back to the Stone Age—or further; the earliest men we know of perceived the things that we perceive. Whether psychology will ever reach the final goal cannot be said; but at any rate the problems are genuine problems; they can be resolved only by intensive and long-continued work; and they demand an extraordinary ingenuity in the devising of experimental controls and an unusual degree of patience in experimenting. Men spend their lives among dead languages and buried cities; why not excavate and explore the inner world of perception?

Let us take an instance or two. Consider, first, the perception of movement by the eye. Many psychologists assume outright a special sensation of movement, something that we might call a travel-sensation. That hypothesis cuts the difficulty; but the sensation is no more admissible than the depth-sensation, and for like reasons. Other psychologists call attention, in a more scientific spirit, to the fact that in all cases of sudden change there is a sensory index of that change. If, for instance, a tone is quickly changed to a higher tone, or a light suddenly reduced to a duller light, there is a moment of sensory blur or confusion, a moment in which the quality or intensity ceases to be clear and distinct; so that, if you were called upon to identify it, you could say only ‘It lies somewhere about such-and-such a part of the scale.’ This blur is the sensory index of change; not a new sensation, but a modification of existing sensation. We have it in the perception of visual movement; there is a blur of positions; and it may reasonably be referred to the positive after-image. A shooting-star flashes across the sky; it leaves a trail of after-image as it moves; you see it both at the place it started from, and at the place where it disappears, all in the same present time; thinking of it, nevertheless, as a star, a point of light like other stars, you perceive movement. The same thing holds for the perception of rapid movement on the skin.

So far everything is in order. Now, however, let us make a simple experiment. You know the stroboscope or zoetrope that is sold in the toy-shops: a cardboard drum, open at the top, that twirls on a handle; a strip of paper, on which are printed phases of some movement (the flight of a bird, the gallop of a horse), is placed inside, round the bottom of the drum; and you look down at the strip, while the instrument revolves, through vertical slits cut at regular intervals in the upper half of the drum-wall; you then see a continuous movement. Suppose that you make a new strip, on which you draw simply two lines, a vertical and a horizontal; you draw them some distance apart, but in such wise that, if they came together, they would form a right-angle. Turn the drum slowly, and you see the two lines; turn it swiftly, and you see the right-angle, like a letter L; turn it at a middle rate, and you see—according to the direction of turn—the vertical fall over into the horizontal, or the horizontal rise up into the vertical. You see movement, where there is no movement to see! Here, then, is a case of perception of movement in terms of sheer brain-habit, of a settled nervous disposition that now has no mental correlate, but whose establishment has depended on the past history of the individual, possibly of the race.

Take, as a second instance, the perception of melody. Primitive melodies seem to be of two types. In the one, the scale arises by synthesis of small tone-steps or tone-distances, which are approximately ‘whole tones’; the melody consists only of two or three of these steps, and the last and lowest tone is the principal note of the tune. In the other, the scale arises by analysis of the larger consonant intervals, fourth and fifth; these intervals are broken up into smaller steps; the octave appears as a drone-bass; the first and highest tone is the principal note. An intermediate type keeps for the most part to small steps, but shows ascents and descents portamento through octave, fifth and fourth; it, too, makes the first and highest tone the principal note. We can account for a good deal of this development: we know that the voice cannot be evenly sustained in recitative, but naturally drops; we have reason to believe that the memory of absolute pitch is strongly developed in primitive peoples (parrots repeat their tunes at the same pitch, and the same thing is largely true of young children); we know the recurrent tonality of the octave (p. 52); we know that the fourth is the natural drop of the voice at the end of a sentence, and the fifth its natural rise in asking a question; we know that men, women and boys, singing in ‘unison,’ will really sing in octaves, and often in fifths and fourths; we know that the semitone, the final unit of our own scales, is the smallest tone-step that can be accurately sung; we know that musical instruments were invented very early, and that they must have helped to give stability to the vocal scale. These things, however, are not enough. For behind all music lies what we must call an intent to express, as behind all speech lies an intent to communicate; and this intent baffles us; we can only say, once again, that it is carried by some native and ingrained disposition of the nervous system. The possibility of music is further bound up with the possibility of transposition; the melody must be reproducible and recognisable, whatever note it start from; and primitive melodies do in fact begin on different notes, and yet keep the same form. It may be that the primitive singer felt his tones, felt the adjustment of his larynx, more keenly than we do. Movements of the larynx are muscular contractions, and their sensations are subject to Weber’s law (p. 68); so that, whether the vocal cords are slack or tense, their tension must be increased in the same proportion to get equal differences in muscular sensation. Here is a possible organic basis for the relative constancy of the tones within a melody; the difficulty is that even primitive melodies seem to be shaped, not by feel, but by ear.

We may take, as a third instance, a group of perceptions that have been named optical illusions. In a certain sense, most of our space-perceptions are illusory. Distance, for example, soon closes up on itself; if we try to stop, halfway, a friend who is walking down a long corridor, we shall be likely to call out before he has gone more than a third of its length. Size is illusory; the size of the moon in the sky is that of a pea held at arm’s length before the eyes. Form is illusory: how often do we see a table square? Only direction is adequately perceived. Yet we do not, somehow, think of all these things as illusions; we are used to them, and can make allowance for them.

There are, on the other hand, certain simple arrangements of dots and lines that yield, in perception, a result markedly different from that which measurement would lead us to expect. These figures have, in recent years, been made the subject of detailed study; that which is here shown has, in particular, been repeatedly discussed and variously explained. The simplicity of the forms is, indeed, treacherous and misleading; analysis is very difficult; and there is no present prospect that investigators will agree.