The Principles of Psychology, Vols. 1-2

* Maybe the localization of intracranial pain is itself due to such association as this of local signs with each other, rather than to their qualitative similarity in neighboring parts {supra, p. 19); though it is conceivable that association and similarity itself should here have one and the same neural basis. If we suppose the sensory nerves from those parts of the body beneath any patch of skin to terminate in the same sensorial braintract as those from the skin itself, and if the excitement of any one fibre tends to irradiate through the whole of that tract, the feelings of all fibres going to that tract would presumabl}^ both have a similar intrinsic quality, and at the same time tend each to arouse the other. Since the same nervetrunk in most cases supplies the skin and the parts beneath, the anatomical hypothesis presents nothing improbable.

than ideally, we have the interval given with its ends, in the form of a single extended object felt. E. H. Weber, in the famous article in which he laid the foundations of all our accurate knowledge of these subjects, laid it down as the logical requisite for the perception of two separated points, that the mind should, along with its conscioicsn^ss of them, become aware of an unexcited interval as suxih. I have only tried to show how the known laws of experience may cause this requisite to he fulfilled. Of course, if the local signs of the entire region offer but little qualitative contrast inter se, the line suggested will be but dimly defined or discriminated in length or direction from other possible lines in its neighborhood. This is what happens in the back, where consciousness can sunder two spots, whilst only vaguely apprehending their distance and direction apart.

The relation of position of the two points is the suggested interval or line. Turn now to the simplest case, that of a single excited spot. How can it suggest its position ? Not by recalling any particular line unless experience have constantly been in the habit of marking or tracing some one line from it towards some one neighboring point. Now on the back, belly, viscera, etc., no such tracing habitually occurs. The consequence is that the only suggestion is that of the whole neighboring circle ; i.e., the spot simply recalls the general region in which it happens to lie. By a process of successive construction, it is quite true that we can also get the feeling of distance between the spot and some other particular spot. Attention, by reinforcing the local sign of one part of the circle, can awaken a new circle round this part, and so de proche en proche we may slide our feeling down from our cheek, say, to our foot. But when we first touched our cheek we had no consciousness of the foot at all.* In the extremities, the lips, the tongue and other mobile parts, the case is different. We there have an instinctive tendency, when a part of lesser discriminative

* Unless, indeed, the foot happen to be spontaneously tingling or something of the sort at the moment. The whole surface of the body is always in a state of semi-conscious irritation which needs only the emphasis of attention, or of some accidental inward irritation, to become strong at any point. sensibility is touched, to move the member so that the touching object glides along it to the place where sensibility is greatest. If a body touches our hand we move the hand over it till the finger-tips are able to explore ii If the sole of our foot touches anything we bring it towards the toes, and so forth. There thus arise lines of habitual passage from all points of a member to its sensitive tip. These are the lines most readily recalled when any point is touched, and their recall is identical with the consciousness of the distance of the touched point from the ' tip.' I think anyone must be aware when he touches a point of his hand or wrist that it is the relation to the finger-tips of which he is usually most conscious. Points on the forearm suggest either the finger-tips or the elbow (the latter being a spot of greater sensibility*). In the foot it is the toes, and so on. A point can only be cognized in its relations to the entire body at once by awakening a visual image of the whole body. Such awakening is even more obviously than the previously considered cases a matter of pure association.

This leads us to the eye. On the retina the fovea and the yellow spot about it form a focus of exquisite sensibility, towards which every impression falling on an outlying portion of the field is moved by an instinctive action of the muscles of the eyeball. Few persons, until their attention is called to the fact, are aware how almost impossible it is to keep a conspicuous visible object in the margin of the field of view. The moment volition is relaxed we find that without our knowing it our eyes have turned so as to bring it to the centre. This is why most persons are unable tc keep the eyes steadily converged upon a point in space with nothing in it. The objects against the walls of the room

* It is true that the inside of the forearm, though its discriminative sensibility is often less than that of the outside, usually rises very prominently into consciousness when the latter is touched. Its cpsthetic sensibility to contact is a good deal finer. We enjoy stroking it from the extensor to the flexor surface around the ulnar side more than in the reverse direction. Pronating movements give rise to contacts in this order, and are frequently indulged in when the back of the fore-arm feels an object against it.

invincibly attract the fovese to themselves. If we contemplate a blank wall or sheet of paper, we always observe in a moment that we are directly looking at some speck upon it which, unnoticed at first, ended by 'catching our eye.' Thus whenever an image falling on the point P of the retina excites attention, it more habitually moves from that point towards the fovea than in any one other direction. The line traced thus by the image is not always a straight line. When the direction of the point from the fovea is neither vertical nor horizontal but oblique, the line traced is often a curve, with its concavity directed upwards if the direction is upwards, downwards if the direction is downwards. This may be verified by anyone who will take the trouble to make a simple experiment with a luminous body like a candle-flame in a dark enclosure, or a star. Gazing first at some point remote from the source of light, let the eye be suddenly turned full upon the latter. The luminous image will necessarily fall in succession upon a continuous series of points, reaching from the one first affected to the fovea. But by virtue of the slowness with which retinal excitements die away, the entire series of points will for an instant be visible as an after-image, displaying the above peculiarity of form according to its situation.''^ These radiating lines are neither regular nor invariable in the same person, nor, probably, equally curved in different individuals. We are incessantly drawing them between the fovea and every point of the field of view. Objects remain in their peripheral indistinctness only so long as they are unnoticed. The moment we attend to them they grow distinct through one of these motions— which leads to the idea prevalent among uninstructed persons that we see distinctly all parts of the field of view at once.

The result of this irwessant tracing of radii is that whenever a local sign P is awakened by a spot of light falling upon itf it recalls forthwith, even though the eyeball be unmoved, the local signs of all the other points ivhich lie between P and the fovea. It recalls them in imaginary form, just as the normal reflex movement would recall them in vivid form ; and with their recall is given a consciousness more or less * These facts were first noticed by Wundt: see his Beitrage, p, 140, 203 See also Lamansky, Pfltiger's Archiv, xi. 418.

faint of the whole line on which they lie. In other words, no ray of light can fall on any retinal spot without the local sign of that spot revealing to us, by recalling the line of its most habitual associates, its direction and distance from the centre of the field. The fovea acts thus as the origin of a system of polar co-ordinates, in relation to which each and every retinal point has through an incessantly-repeated process of association its distance and direction determined. Were P alone illumined and all the rest of the field dark we should still, even with motionless eyes, know whether P lay high or low, right or left, through the ideal streaky different from all other streaks, which P alone has the power of awakening.*

* So far all has been plain sailing, but our course begins to be so tortuous when we descend into minuter detail that I will treat of the more precise determination of locality in a long note. When P recalls an ideal line leading to the fovea the line is felt in its entirety and but vaguely ; whilst P, which we supposed to be a single star of actual light, stands out in strong distinction from it. The ground of the distinction between P and the ideal line which it terminates is manifest— P being vivid while the line is faint ; hut why should P Twld the particular 'position it does, at the end of the line, rather than anywhere else— for exampU, in its middle? That seems something not at all manifest.

To clear up our thoughts about this latter mystery, let us take the case of an actual line of light, none of whose parts is ideal. The feeling of the line is produced, as we know, when a multitude of retinal points are excited together, each of which when excited separately would give rise to one of the feelings called local signs. Each of these signs is the feeling of a small space. From their simultaneous arousal we might well suppose a feeling of larger space to result. But why is it necessary that in this larger spaciousness the sign a should appear always at one end of the line, z at the other, and m in the middle ? For though the line be a unitary streak of light, its several constituent points can nevertheless break out from it, and become alive, each for itself, under the selective eye of attention.

The uncritical reader, giving his first careless glance at the subject, will say that there is no mystery in this, and that * of course ' local signs must appear alongside of each other, each in its own place; — there is no other way possible. But the more philosophic student, whose business it is to discover difficulties quite as much as to get rid of them, will reflect that it is conceivable that the partial factors might fuse into a larger space, and yet not each be located within it any more than a voice is located in a chorus He will wonder how, after combining into the line, the points can become severally alive again : the separate pufCs of a ' sirene ' no longer strike the ear after they have fused into a certain pitch of sound. He will recall the fact that when, after looking at things with one eye closed, we

And with this we can close the first great division oi our subject. We have shown that, within the range of double, by opening the other eye, the number of retinal points affected, the new retinal sensations do not as a rule appear alongside of the old ones and additional to them, but merely make the old ones seem larger and nearer. Why should the affection of new points on the same retina have so different a result? In fact, he will see no sort of logical connection between (1) the original separate local signs, (2) the line as a unit, (3) the line with the points discriminated in it, and (4) the various nerve-processes which subserve all these different things. He will suspect our local sign of being a very slippery and ambiguous sort of creature. Positionless at first, it no sooner appears in the midst of a gang of companions than it is found maintaining the strictest position of its own, and assigning place to each of its associates. How is this possible V Must we accept what we rejected a while ago as absurd, and admit the points each to have position in se ? Or must we suspect that our whole construction has been fallacious, and that we have tried to conjure up, out of association, qualities which the associates never contained?

There is no doubt a real difficulty here; and the shortest way of dealing with it would be to confess it insoluble and ultimate. Even if position be not an intrinsic character of any one of those sensations we have called local signs, we must still admit that there is something about every one of them that stands for the potentiality of position, and is the ground why the local sign, when it gets placed at all, gets placed 7iere rather than there. If this ' something ' be interpreted as a physiological something, as a mere nerveprocess, it is easy to say in a blank way that when it is excited alone, it is an ' ultimate fact ' (1) that a positionless spot will appear; that when it is excited together with other similar processes, but without ,the process of discriminative attention, it is another 'ultimate fact ' (2) that a unitary line will come; and that the final 'ultimate fact' (3) is that, when the nerveprocess is excited in combination with that other process which subserves the feeling of attention, what results will be the line with the local sign inside of it determined to a particular place. Thus we should escape the responsibility of explaining, by falling back on the everlasting inscrutability of the psycho-neural nexus. The moment we call the ground of localization physiological, we need only point out how, in those cases in which localization occurs, the physiological process differs from those in which it does not, to have done all we can possibly do in the matter. This would be unexceptionable logic, and with it we might let the matter drop, satisfied that there was no self-contradiction in it, but only the universal psychological puzzle of how a new mode of consciousness emerges whenever a fundamentally new mode of nervous action occurs.

But, blameless as such tactics would logically be on our part, let us see whether we cannot push our theoretic insight a little farther. It seems to me we can. We cannot, it is true, give a reason why the line we feel when process (2) awakens should have its own peculiar shape; nor can we explain the essence of the process of discriminative attention. But we can see why, if the brute facts be admitted that a line may have one of its parts singled out by attention at all, and that that part may appear in relation to

every sense, experience takes ah initio the spatial form. We have also shown that in the cases of the retina and skin other parts at all, the relation must be in the line itself, — for the line and the parts are the only things supposed to be in consciousness. And we can furthermore suggest a reason why parts appearing thus in relation to each other in a line should fall into an immutable order, and each within that order keep its characteristic place.

If a lot of such local signs all have any quality which evenly augments as we pass from one to the other, we can arrange them in an ideal serial order, in which any one local sign must lie below those with more, above those with less, of the quality in question. It must divide the series into two parts, — unless indeed it have a maximum or minimum of the quality, when it either begins or ends it. Such an ideal series of local signs in the mind is, however, not yet identical with the feeling of a line in space. Touch a dozen points on the skin successively, and there seems no necessary reason why the notion of a definite line should emerge, even though we be strongly aware of a gradation of quality among the touches. We may of course symbolically arrange them in a line in our thought, but we can always distinguish between a line symbolically thought and a line directly felt.

But note now the peculiarity of the nerve-processes of all these local signs: though they may give no line when excited successively, when excited together they do give the actual sensation of a line in space. The sum of them is the neural process of that line; the sum of their feelings is the feeling of that line; and if we begin to single out particular points from the line, and notice them by their rank, it is impossible to see how this rank can appear except as an actual fixed space-position sensibly felt as a bit of the total line. The scale itself appearing as a line, rank in it must appear as a definite part of the line. If the seven notes of an octave, when heard together, appeared to the sense of hearing as an outspread line of sound — which it is needless to say they do not — why then no one note could be discriminated without being localized, according to its pitch, in the line, either as one of its extremities or as some part between.

But not alone the gradation of their quality arranges the local-sign feelings in a scale. Our momments arrange them also in a ^me-scale. Whenever a stimulus passes from point a of the skin or retina to point/, it awakens the local-sign feelings in the perfectly definite time-order abcdef. It cannot excite/ until cde have been successively aroused. The feeling c sometimes is preceded by ah, sometimes followed by ha, according to the movement's direction; the result of it all being that we never feel either a, c, or/ without there clinging to it faint reverberations of the various timeorders of transition in which, throughout past experience, it has been aroused. To the local sign a there clings the tinge or tone, the penumbra or fringe, of the transition hcd. To/ to c, there cling quite different tones. Once admit the principle that a feeling may be tinged by the reproductive consciousness of an habitual transition, even when the transition is not made, and it seems entirely natural to admit that, if the transition be habitually in the order abcdef, and if a, c, and/ be felt separately at all, a will be felt with an essential earliness, /with an essential lateness, and that e will

every sensible total may be subdivided by discriminative attention into sensible parts, which are also spaces, and into relations between the parts, these being sensible spaces too. Furthermore, we have seen (in a foot-note) that different parts, once discriminated, necessarily fall into a determinate order, both by reason of definite gradations in their quality, and by reason of the fixed order of time-succession in which movements arouse them. But in all this nothing has been said of the comparative measurement of one sensible space-total against another, or of the way in which, by summing our divers simple sensible spaceexperiences together, we end by ronstructing what we regard as the unitary, continuous, and infinite objective Space of the real world. To this more difficult inquiry we next pass.

(1) How is the subdivision and measurement of the several sensorial spaces completely effected? and (2) How do their mutual addition and fusion and reduction to the same scale, in a ivord, how does their synthesis, occur ? I think that, as in the investigation just finished, we found ourselves able to get along without invoking any data but those that pure sensibility on the one hand, and the ordinary intellectual powers of discrimination and recollec-

fall between. Thus those psychologists who set little store by local signs and great store by movements in explaining space-perception, would have a perfectly definite time-order, due to motion, by which to account for the definite order of positions that appears when sensitive spots are excited all at once. Without, however, the preliminary admission of the ' ultimate fact ' that this collective excitement shall feel like a line and nothing else, it can never be explained why the new order should needs be an order of positions, and not of merely ideal serial rank. We shall hereafter have any amount of opportunity to observe how thoroughgoing is the participation of motion in all our spatial measurements. Whether the local signs have their respective qualities evenly graduated or not, the feelings of transition must be set down as among the verm causce in localization. But the gradation of the local signs is hardly to be doubted; so we may believe ourselves really to possess two sets of reasons for localizing any point we may happen to distinguish from out the midst of any line or any larger space.

fcion on the other, were able to yield; so here we shall emerge from our more complicated quest with the conviction that all the facts can be accounted for on the supposition that no other mental forces have been at work save those we find everywhere else in psychology : sensibility, namely, for the data ; and discrimination, association^ memory, and choice for the rearrangements and combina tions which they undergo. How are spatial subdivisions brought to consciousness ? in other words. How does spatial discrimination occur? The general subject of discrimination has been treated in a previous chapter. Here we need only inquire what are the conditions that make spatial discrimination so much finer in sight than in touch, and in touch than in hearing, smell, or taste.

The first great condition is, that different points of the surface shall differ in the quality of their immanent sensibility ^ that is, that each shall carry its special local-sign. If the skin felt everywhere exactly alike, a foot-bath could be distinguished from a total immersion, as being smaller, but never distinguished from a wet face. The local-signs are indispensable ; two points which have the same localsign will always be felt as the same point. We do not judge them two unless we have discerned their sensations to be different."^ Granted none but homogeneous irritants, that organ would then distinguish the greatest multiplicity of irritants — would count most stars or compass-points, or best compare the size of two wet surfaces — whose local sensibility was the least even. A skin whose sensibility shaded rapidly off from a focus, like the apex of a boil, would be better than a homogeneous integument for spatial perception. The retina, with its exquisitely sensitive fovea, has this peculiarity, and undoubtedly owes to it a great part

* M. Binet (Revue Philosophique, Sept. 1880, page 291) says we judge them locally different as soon as their sensations differ enough for us to distinguish them as qualitatively different when successively excited. Thii \& not strictly true. Skinsensations, different enough to be discriminated »rhen sueeesiiw, may still fuse locally if excited both at once. of the minuteness with which we are able to subdivide the total bigness of the sensation it yields. On its periphery the local differences do not shade off very rapidly, and we can count there fewer subdivisions.

But these local differences of feeling , so long as the surface is unexcited from without , are almost null. I canot feel them by a pure mental act of attention unless they belong to quite distinct parts of the body, as the nose and the lip, the fingertip and the ear ; their contrast needs the reinforcement of outward excitement to be felt. In the spatial muchness of a colic — or, to call it by the more spacious-sounding vernacular, of a * bellyache ' — one can with difficulty distinguish the north-east from the south-west corner, but can do so much more easily if, by pressing one's finger against the former region, one is able to make the pain there more intense.

The local differenjces require then an adventitious sensation, superinduced upon them, to awaken the attention. After the attention has once been awakened in this way, it may continue to be conscious of the unaided difference ; just aa a sail on the horizon may be too faint for us to notice until someone's finger, placed against the spot, has pointed it out to us, but may then remain visible after the finger has been withdrawn. But all this is true only on condition that separate points of the surface may be exclusively stimulated. If the whole surface at once be excited from without, and homogeneously, as, for example, by immersing the body in salt water, local discrimination is not furthered. The localsigns, it is true, all awaken at once ; but in such multitude that no one of them, with its specific quality, stands out in contrast with the rest. If, however, a single extremity be immersed, the contrast between the wet and dry parts is strong, and, at the surface of the water especially, the localsigns attract the attention, giving the feeling of a ring surrounding the member. Similarly, two or three wet spots separated by dry spots, or two or three hard points against the skin, will help to break up our consciousness of the latter's bigness. In cases of this sort, where points receiving an identical kind of excitement are, nevertheless, felt to be locally distinct, and the objective irritants are also

judged multiple, — e.g., compass-points on skin or stars on retina, — the ordinary explanation is no doubt just, and we judge the outward causes to be multiple because we have discerned the local feelings of their sensations to be different. Capacity for partial stirmdation is thus the second condition favoring discrimination. A sensitive surface which has to be excited in all its parts at once can yield nothing but a sense of undivided largeness. This appears to be the case with the olfactory, and to all intents and purposes with the gustatory, surfaces. Of many tastes and flavors, even simultaneously presented, each affects the totality of its respective organ, each appears with the whole vastness given by that organ, and appears interpenetrated by the rest.*

* It may, however, be said that even iu the tongue there is a determination of bitter flavors to the back and of acids to the front edge of the organ. Spices likewise affect its sides and front, and a taste like that of alum localizes itself, by its styptic effect on the portion of mucous membrane, which it immediately touches, more sharply than roast pork, for example, which stimulates all parts alike. The pork, therefore, tastes more spacious than the alum or the pepper. In the nose, too, certain smells, of which vinegar may be taken as the type, seem less spatially extended than heavy, suffocating odors, like musk. The reason of this appears to be that the former inhibit inspiration by their sharpness, whilst the latter are drawn into the lungs, and thus excite an objectively larger surface. The asci* p- tion of height and depth to certain notes seems due, not to any localization of the sounds, but to the fact that a feeling of vibration in the chest and tension in the gullet accompanies the singing of a bass note, whilst, when we sing high, the palatine mucous membrane is drawn upon by the muscles which move the larynx, and awakens a feeling in the roof of the mouth.

The only real objection to the law of partial stimulation laid down in the text is one that might be drawn from the organ of hearing; for, according to modern theories, the cochlea may have its separate nerve-termini exclusively excited by sounds of differing pitch, and yet the sounds seem all to fill a common space, and not necessarily to be arranged alongside of each other. At most the high note is felt as a thinner, brighter streak against a darker background. In an article on Space, published in the Journal of Speculative Philosophy for January, 1879, 1 ventured to suggest that possibly the auditory nerve termini might be "excited all at once by sounds of any pitch, as the whole retina would be by every luminous point if there were no dioptric apparatus affixed." And I added : " Notwithstanding the brilliant conjectures of the last few years which assign differ, ent acoustic end-organs to different rates of air-wave, we are still greatly in the dark about the subject ; and I, for my part, would much more confidently reject a theory of hearing which violated the principles advanced in