The Principles of Psychology, Vols. 1-2

*'The attentive observation of a person born blind . . . has convinced me that the sense of touch by itself is altogether incompetent to afford us the representation of extension and space. ... In fact, to those born blind, time serves instead of space. Vicinity and distance mean in their mouths nothing more than the shorter or longer time . . . necessary to attain from some one feeling to some other." After my own observation of blind people, I should hardly have considered this as anything but an eccentric opinion, worthy to pair off with that other belief that color is primitively seen without extent, had it not been for the remarkable Essay on Tactile and Visual Space by M. Ch. Dunan, which appeared in the Eevue Philosophique for 1888. This author quotes * three very competent witnesses, all officials in institutions for the blind [it does not appear from the text that more than one of them was blind himself], who say that blind people only live in time. M. Dunan himself does not share exactly this belief, but he insists that the blind man's and the seeing man's representation of space have absolutely naught in common, and that we are deceived into believing that what they mean by space is analogous to what we mean, by the fact that so many of them are but semi-blind and still think in visual terms, and from the farther fact that they all talk in visual terms just like ourselves. But on examining M. Dunan's reasons one finds that they all rest on the groundless logical assumption that the perception of a geometrical form which we get with our eyes, and that which a blind man gets with

his fingers, must either be absolutely identical or absolutely unlike. They cannot be similar in diversity, " for they are simple notions, and it is of the essence of such to enter the mind or leave it all at once, so that one who has a simple notion at all, possesses it in all its completeness. . . . Therefore, since it is impossible that the blind should have of the forms in question ideas completely identical with our seeing ones, it follows that their ideas must be radically different from and ivholly irredvcible to our ownJ" * Hereupon M. Dunan has no difficulty in finding a blind man who still preserves a crude sensation of diffused light, and who says when questioned that this light has no extent. Having * no extent ' appears, however, on farther questioning, to signify merely not enveloping any particular tactile objects, nor being located within their outline; so that (allowing for latitude of expression) the result tallies perfectly with our own view. A relatively stagnant retinal sensation of diffused light, not varying when different objects are handled, would naturally remain an object quite apart. If the word * extent ' were habitually used to denote tactile extent, this sensation, having no tactile associates whatever, would naturally have * extent ' denied of it. And yet all the while it would be analogous to the tactile sensations in having the quality of bigness. Of course it would have no other tactile qualities, just as the tactile objects have no other optical qualities than bigness. All sorts of analogies obtain between the spheres of sensibility. Why are ' sweet ' and * soft ' used so synonymously in most languages ? and why are both these adjectives applied to objects of so many sensible kinds. Bough sounds, heavy smells, hard lights, cold colors, are other examples.

Nor does it follow from such analogies as these that the sensations compared need be composite and have some of their parts identical. We saw in Chapter XIII that likeness and difference are an elementary relation, not to be resolved in every case into a mixture of absolute identity and absolute heterogeneity of content (cf. Vol. I, pp. 492-3). determinations the blind man's space is very different from our space, yet a deep analogy remains between the two. * Big ' and * little,' * far ' and * near,' are similar contents of consciousness in both of us. But the measure of the bigness and the farness is very different in him and in ourselves. He, for example, can have no notion of what we mean by objects appearing smaller as they move away, because he must always conceive of them as of their constant tactile size. Nor, whatever analogy the two extensions involve, should we expect that a blind man receiving sight for the first time should recognize his new-given optical objects by their familiar tactile names. Molyneux wrote to Locke :

" Suppose a man born blind, and now adult, and taught by his touch to distinguish between a cube and a sphere, ... so as to tell, when he felt one and the other, which is the cube, which the sphere. Suppose then the cube and sphere placed on a table an i the blind man to be made to see ; query, whether by his sight, before he touched them, he could now distinguish and tell which is the globe, which the cube ?" This has remained in literature as * Molyneux's query.* Molyneux answered *No.' And Locke says :^

" I agree with this thinking gentleman whom I am proud to call my friend, and am of opinion that the blind man at first sight would not be able to say which was the globe, which the cube, whilst he only saw them ; though he could unerringly name them by his touch and certainly distinguish them by the difference of their figures felt." This opinion has not lacked experimental confirmation. From Chesselden's case downwards, patients operated for congenital cataract have been unable to name at first the things they saw. " So, Puss, I shall know you another time," said Chesselden's patient, after catching the cat, looking at her steadfastly, and setting her down. Some of this incapacity is unquestionably due to general mental confusion at the new experience, and to the excessively unfavorable conditions far perception which an eye with its lens just extirpated affords. That the analogy of inner nature between the retinal and tactile sensations goes beyond mere extensity is proved by the cases where the patients were the most intelligent, as in the young man operated on by Dr. Eranz.

who named circular, triangular, and quadrangular figures at first sight.* It is when we come to analyze minutely the conditions of visual perception that difficulties arise which have made psychologists appeal to new and 5'was^-mythical mental powers. But I firmly believe that even here exact investigation will yield the same verdict as in the cases studied hitherto. This subject will close our survey of the facts ; and if it give the result I foretell, we shall be in the best of positions for a few final pages of critically historical review.

If a common person is asked how he is enabled to see things as they are, he will simply reply, by opening his eyes and looking. This innocent answer has, however, long since been impossible for science. There are various paradoxes and irregularities about what we appear to perceive under seemingly identical optical conditions, which immediately raise questions. To say nothing now of the time-honored conundrums of why we see upright with an inverted retinal picture, and why we do not see double ; and to leave aside the whole field of color-contrasts and ambiguities, as not directly relevant to the space-problem, — it is certain that the same retinal image makes us see quite differently-sized and differently-shaped objects at different times, and it is equally certain that the same ocular movement varies in its perceptive import. It ought to be possible, were the act of perception completely and simply intelligible, to assign for every distinct judgment of size, shape, and position a distinct optical modification of some kind as its occasion. And the connection between the two ought to be so constant that, given the same modification, we should always have the same judgment. But if we

* Philosophical Transactions, 1841. In T. K. Abbot's Sight and Touch there is a good discussion of these cases. Obviously, positive cases are oi more importance than negative. An under-witted peasant, Noe M., whose case is described by Dr. Dufour of Lausanne (Guerison d'un Aveugle nC; 1876) is much made of by MM. ISaville and Dunan ; but it seems to me only to show how little some people can deal with new experiences in which others find themselves quickly at home Tliis man could not even tell whether one of his first objects of sight moved or stood still (p. 9).

study the facts closely we soon find no such constant conneciion hettveen either jitdgment and retinal modification, or htdgment and muscular modification, to exist. The judgment ^eems to result from the combination of retinal, muscular and intellectual factors with each other ; and any one of them may occasionally overpower the rest in a way which «eems to leave the matter subject to no simple law. The scientific study of the subject, if we omit Descartes, began with Berkeley, and the particular perception he analyzed in his New Theory of Vision was that of distance or depth. Starting with the physical assumption that a difference in the distance of a point can make no difference in the nature of its retinal image, since "distance being a line directed endwise to the eye, it projects only one point in the fund of the eye — which point remains invariably the same, whether the distance be longer or shorter," he concluded that distance could not possibly be a visual sensation, but must be an intellectual * suggestion ' from * custom ' of some nonvisual experience. According to Berkeley this experience was tactile. His whole treatment of the subject was excessively vague, — no shame to him, as a breaker of fresh ground, — but as it has been adopted and enthusiastially hugged in all its vagueness by nearly the whole line of British psychologists who have succeeded him, it will be well for us to begin our study of vision by refuting his notion that depth cannot possibly be perceived in terms of purely visual feeling.

Berkeleyans unanimously assume that no retinal sensa. tion can primitively be of volume ; if it be of extension at all (which they are barely disposed to admit), it can be only of two-, not of three-, dimensional extension. At the beginning of the present chapter we denied this, and adduced facts to show that all objects of sensation are voluminous in three dimensions (cf. p. 136 ff.). It is impossible to lie on one's back on a hill, to let the empty abyss of blue fill one's whole visual field, and to sink deeper and deeper into the merely sensational mode of consciousness regarding it, without feeling that an indeterminate, palpitating, circling

depth is as indefeasibly one of its attributes as its breadth. We may artificially exaggerate this sensation of depth. Bise and look from the hill-top at the distant view ; represent to yourself as vividly as possible the distance of the uttermost horizon ; and then with inverted head look at the same. There will be a startling increase in the perspective, a most sensible recession of the maximum distance ; and as you raise the head you can actually see the horizonline again draw near.*

Mind, I say nothing as yet about our estimate of the real ' amount of this depth or distance. I only want to confirm its existence as a natural and inevitable optical consort of the two other optical dimensions. The field of view is always a volume-miii. Whatever be supposed to be its absolute and * real ' size, the relative sizes of its dimensions are functions of each other. Indeed, it happens perhaps most often that the breadth- and height-feeling take their absolute measure from the depth-feeling. If we plunge our head into a wash-basin, the felt nearness of the bottom makes us feel the lateral expanse to be small. If, on the contrary, we are on a mountain-top, the distance of the horizon carries with it in our judgment a proportionate

* What may be the physiological process connected with this increased sensation of depth is hard to discover. It seems to have nothing to do with the parts of the retina affected, since the mere inversion of the picture (bymirrors, reflecting prisms, etc.), without inverting the head, does not seem to bring it about ; nothing with sympathetic axial rotation of the eyes, which might enhance the perspective through exaggerated disparity of the two retinal images (see J. J. Mtiller, ' Raddrehung u. Tiefendimension,' Leipzig Acad. Berichte, 1875, page 124), for one-eyed persons get it as strongly as those with two eyes. I cannot find it to be connected with any alteration in the pupil or with any ascertainable strain in the muscles of the eye, sympathizing with those of the body. The exaggeration of distance is even greater when we throw the head over backwards and contract our superior recti in getting the view, than when we bend forward and contract the inferior recti. Making the eyes diverge slightly by weak prismatic glasses has no such effect. To me, and to all whom I have asked to repeat the observation, the result is so marked that I do not well understand how such an observer as Helmholtz, who has carefully examined vision with inverted head, can have overlooked it. (See hie Phys. Optik, pp. 433, 723, 728. 772.) I cannot help thinking that anyone who can explain the exaggeration of the depth-sensation in this case wiU at the same time throw much light on its normal constitution.

height and length in the mountain-chains that bound it to our view. But as aforesaid, let us not consider the question of absolute size now, — it must later be taken up in a thorough way. Let us confine ourselves to the way in which the three dimensions which are seen, get their values fixed relatively to each other. Eeid, in his Inquiry into the Human Mind, has a section * Of the Geometry of Visibles,' in which he assumes to trace what the perceptions would be of a race of * Idomenians ' reduced to the sole sense of sight. Agreeing with Berkeley that sight alone can give no knowledge of the third dimension, he humorously deduces various ingenious absurdities in their interpretations of the material appearances before their eyes.

Now I firmly believe, on the contrary, that one of Eeid's Idomenians would frame precisely the same conception of the external world that we do, if he had our intellectual powers.* Even were his very eyeballs fixed and not movable like ours, that would only retard, not frustrate, his education. For the same object, by alternately covering in its lateral movements different parts of his retina, would determine the mutual equivalencies of the first two dimensions of the field of view ; and by exciting the physiological cause of his perception of depth in various degrees, it would establish a scale of equivalency between the first two and the third.

First of all, one of the sensations given by the object is chosen to represent its * real * size and shape, in accordance with the principles laid down on pp. 178 and 179. One sensation measures the ' thing ' present, and the * thing ' then measures the other sensations. The peripheral parts of the retina are equated with the central by receiving the image of the same object. This needs no elucidation in case the * " In Froriep's Notizen (1838, July), No. 133, is to be found a detailed account, with a picture, of an Esthonian girl, Eva Lauk, then fourteen years old, born with neither arms nor legs, which concludes with the following words : ' According to the mother, her intellect developed quite as fast as that of her brother and sisters ; in particular, she came as quickl]^ to a right judgment of the size and distance of visible objects, although, of courge, she had no use of hands.' " (Schopenhauer, Welt als Wille, ii

object does not change its distance or its front. But suppose, to take a more complicated case, that the object is a stick, seen first in its whole length, and then rotated round' one of its ends ; let this fixed end be the one near the eye. lu this movement the stick's image will grow progressively shorter ; its farther end will appear less and less separated laterally from its fixed near end ; soon it will be screened by the latter, and then reappear on the opposite side, and finally on that side resume its original length. Suppose this movement to become a familiar experience ; the mind will presumably react upon it after its usual fashion (which is that of unifying all data which it is in any way possible to unify), and consider it the movement of a constant object rather than the transformation of a fluctuating one. Now, the sensation of depth which it receives during the experience is awakened more by the far than by the near end of the object. But how much depth ? What shall measure its amount ? Why, at the moment the far end is ready to be eclipsed, the difference of its distance from the near end's distance must be judged equal to the stick's whole length ; but that length has already been judged equal to a certain optical sensation of breadth. Thus we find that given amounts of the visual depth-feeling become signs of fixed amounts of the visual breadth-feeling. The measurement of distance is, as Berkeley truly said, a result of suggestion and experience. But visual experience alone is adequate to produce ity and this he erroneously denied.

Suppose a colonel in front of his regiment at dressparade, and suppose he walks at right angles towards the midmost man of the line. As he advances, and surveys the line in either direction, he looks more and more doum it and less and less at it, until, when abreast of the midmost man, he feels the end men to be most distant ; then when the line casts hardly any lateral image on his retina at all, what distance shall he judge to be that of the end men? Why, half the length of the regiment as it was originally seen, of course ; but this length was a moment ago a retinal object spread out laterally before his sight. He has now merely equated a retinal depth -feeling with a retinal breadth-feeling. If the regiment moved, and the

colonel stood still, the result would be the same. In sucli ways as these a creature endowed with eyes alone could hardly fail of measuring out all three dimensions of the space he inhabited. And we ourselves, I think, although we may often ' realize ' distance in locomotor terms (as Berkeley says we must always do), yet do so no less often in terms of our retinal map, and always in this way the more spontaneously. Were this not so, the three visual dimensions could not possibly feel to us as homogeneous as they do, nor as commensurable inter se.

Let us then admit distance to be at least as genuinely optical a content of consciousness as either height or breadth. The question immediately returns. Can any of them be said in any strictness to be optical sensations ? We have contended all along for the affirmative reply to this question, but must now cope with difficulties greater than any that have assailed us hitherto. A sensation is, as we have seen in Chapter XVII, the mental affection that follows most immediately upon the stimulation of the sense tract. Its antecedent is directly physical, no psychic links, no acts of memory, inference, or association intervening. Accordingly, if we suppose the nexus between neural process in the sense-organ, on the one hand, and conscious affection, on the other, to be by nature uniform, the same process ought always to give the same sensation ; and conversely, if what seems to be a sensation varies whilst the process in the sense-organ remains unchangedf the reason is presumably that it is really not a sensation but a higher mental product, whereof the variations depend on events occurring in the system of higher cerebral centres.

Now the size of the field of view varies enormously in all three dimensions, without our being able to assign with any definiteness the process in the visual tract on which the variation depends. We just saw how impossible such assignment was in the case where turning down the head produces the enlargement. In general, the maximum feeling of depth or distance seems to take the lead in determining the apparent magnitude of the whole field, and the

two other dimensions seem to follow. If, to use the former instance, I look close into a wash-basin, the lateral extent of the field shrinks proportionately to its nearness. If I look from a mountain, the things seen are vast in height and breadth, in proportion to the farness of the horizon. But when we ask what changes in the eye determine how great this maximum feeling of depth or distance (which is undoubtedly felt as a unitary vastness) shall be, we find ourselves unable to point to any one of them asbein^ its absolutely regular concomitant. Convergence, accommodation, double and disparate images, differences in the parallactic displacement when we move our head, faintness of tint, dimness of outline, and smaliness of the retinal image of objects named and known, are all processes that have something to do with the perception of * far ' and of * near ' ; but the effect of each and any one of them in determining such a perception at one moment may at another moment be reversed by the presence of some other sensible quality in the object, that makes us, evidently by reminding us of past experience, judge it to be at a different distance and of another shape. If we paint the inside of a pasteboard-mask like the outside, and look at it with one eye, the accommodation- and parallax-feelings are there, but fail to make us see it hollow, as it is. Our mental knowledge of the fact that human faces are always convex overpowers them, and we directly perceive the nose to be nearer to us than the cheek instead of farther of.

The other organic tokens of farness and nearness are proved by similar experiments (of which we shall ere long speak more in detail) to have an equally fluctuating import. They lose all their value whenever the collateral circumstances favor a strong intellectual conviction that the object presented to the gaze is improbable. — cannot be either what or wh^re they would make us perceive it to be. Now the query immediately arises : Can the feelings of tJwse processes in the eye, since they are so easily neutralized and reversed by intellectual suggestions, ever have been direct sensation^ of distance at all ? Ought we not rather to assume, since the distances which we see in spite of them are conclusions from past experience, that the distances which w«

see by means of them are equally such conclusions ? Ought we not, in short, to say unhesitatingly that distance must be an intellectual and not a sensible content of consciousness ? and that each of these eye-feelings serves as a mere signal to awaken this content, our intellect being so framed that sometimes it notices one signal more readily and sometimes another ? " It may be taken for a general rule that things which are produced by custom may be undone or changed by disuse or by contrary custom. On the other hand, it is a strong argument that an effect is not owing to custom, but to the constitution of nature, when a contrary custom is

More briefly, a way of seeing things that can be unlearned was presumably learned, and only what we cannot unlearn is instinctive. This seems to be Helmholtz's view, for he confirms Reid's maxim by saying in emphatic print : " No elements in our perception can be sensational which may be overcome or reversed by factors of demonstrably experimental origin. Whatever can be overcome by suggestions of experience must be regarded as itself a product of experience and custom. If we follow this rule it will appear that only qualities are sensational, whilst almost all spatial attributes are results of habit and experience."*

This passage of Helmholtz's has obtained, it seems to me, an almost deplorable celebrity. The reader will please observe its very radical import. Not only would he, and does he, for the reasons we have just been ourselves considering, deny distance to be an optical sensation ; but, extending the same method of criticism to judgments of size, shape, and direction, and finding no single retinal or muscular process in the eyes to be indissolubly linked with any one of these, he goes so far as to say that all optical space-perceptions whatsoever must have an intellectual

* Physiol. Optik, p. 438. Helmholtz's reservation of * qualities ' is inconsistent. Our judgments of light and color vary as much as our judgments of size, shape, and place, and ought by parity of reasoning to be called intellectual products and not sensations. In other places he does treat color as if it were an intellectual product. origin, and a content that no items of visual sensibility can account for."^ As Wundt and others agree with Helmholtz here, and as their conclusions, if true, are irreconcilable with all the sensationalism which I have been teaching hitherto, it clearly devolves upon me to defend my position against this new attack. But as this chapter on Space is already so overgrown with episodes and details, I think it best to reserve the refutation of their general principle for the next chapter, and simply to assume at this point its untenability. This has of course an arrogant look ; but if the reader will bear with me for not very many pages more, I shall hope to appease his mind. Meanwhile I affirm confidently that tlie same outer objects actually feel different to us according as our brain reacts on them in one way or another by making us 'perceive them as this or as that sort of thing. So true is this that one may well, with Stumpf,t reverse Helmholtz's query, and ask : ""What would become of our sense-perceptions in case experience were not able so to transform them ? " Stumpf adds : " All wrong perceptions that depend on peculiarities in the organs are more or less perfectly corrected by the influence of imagination following the guidance of experience."

If, therefore, among the facts of optical space-perception (which we must now proceed to consider in more detail) we find instances of an identical organic eye-process, giving us different perceptions at different times, in consequence of diff'erent collateral circumstances suggesting different objective facts to our imagination, we must not hastily conclude, with the school of Helmholtz and Wundt, that the organic eye -process pure and simple, without the collateral circumstances, is incapable of giving us any sensation of a spatial kind at all. We must rather seek to discover by what mean^ the circumstances can so have transformed a space-sensation, which, but for their presence, would probably have been felt in its natural purity. And I may as well say

* It is needless at this point to consider what Helmholtz's views of the nature of the intellectual space -yielding process may be. He vacillates— we shall later see how. now in advance that we shall find the means to be nothing more or less than association — the suggestion to the mind of optical objects not actucHly present^ but more habitually associated with the ' collateral circumstances ' than the sensation which they now displace and being imagined now with a quasi-hallucinatory strength. But before this conclusion emerges, it will be necessary to have reviewed the most important facts of optical space-perception, in relation to the organic conditions on which they depend. Eeaders acquainted with German optics will excuse what is already familiar to them in the following section.*

* Before embarking on this new topic it will be well to shelve, once for all, the problem of what is the physiological process that underlies the distance-feeling, Since one-eyed people have it, and are inferior to the two-eyed only in measuring its gradations, it can have no exclusive connection with the double and disparate images produced by binocular parallax. Since people with closed eyes, looking at an after-image, do not usually see it draw near or recede with varying convergence, it cannot be simply constituted by the convergencefeeling. For the same reason it would appear non-identical with the feeling of accommodation. The differences of apparent parallactic movement between far and near objects as we move our head cannot constitute the distance-sensation, for such differences may be easily reproduced experimentally (in the movements of visible spots against a background) without engendering any illusion of perspective. Finally, it is obvious that visible faintness, dimness, and smallness are not per se the feeling of visible distance, however much in the case of well-known objects they may serve as signs to suggest it.