Illusions Part 2

While in the following examination of sense-illusions we put out of sight what certain philosophers say about the illusoriness of perception as a whole, we shall also do well to leave out of account what physical science is sometimes supposed to tell us respecting a constant element of illusion in perception. The physicist, by reducing all external changes to "modes of motion," appears to leave no room in his world-mechanism for the secondary qualities of bodies, such as light and heat, as popularly conceived. Yet, while allowing this, I think we may still regard the attribution of qualities like colour to objects as in the main correct and answering to a real fact. When a person says an object is red, he is understood by everybody as affirming something which is true or false, something therefore which either involves an external fact or is illusory. It would involve an external fact whenever the particular sensation which he receives is the result of a physical action (other vibrations of a certain order), which would produce a like sensation in anybody else in the same situation and endowed with the normal retinal sensibility. On the other hand, an illusory attribution of colour would imply that there is no corresponding physical agency at work in the case, but that the sensation is connected with exceptional individual conditions, as, for example, altered retinal sensibility.

We are now, perhaps, in a position to frame a rough definition of an illusion of perception as popularly understood. A large number of such phenomena may be described as consisting in the formation of percepts or quasi-percepts in the minds of individuals under external circ.u.mstances which would not give rise to similar percepts in the case of other people.

A little consideration, however, will show that this is not an adequate definition of what is ordinarily understood by an illusion of sense.

There are special circ.u.mstances which are fitted to excite a momentary illusion in all minds. The optical illusions due to the reflection and refraction of light are not peculiar to the individual, but arise in all minds under precisely similar external conditions.

It is plain that the illusoriness of a perception is in these cases determined in relation to the sense-impressions of other moments and situations, or to what are presumably better percepts than the present one. Sometimes this involves an appeal from one sense to another. Thus, there is the process of verification of sight by touch, for example, in the case of optical images, a mode of perception which, as we have seen, gives a more direct cognition of external quality. Conversely, there may occasionally be a reference from touch to sight, when it is a question of discriminating two points lying very close to one another. Finally, the same sense may correct itself, as when the illusion of the stereoscope is corrected by afterwards looking at the two separate pictures.

We may thus roughly define an illusion of perception as consisting in the formation of a quasi-percept which is peculiar to an individual, or which is contradicted by another and presumably more accurate percept.

Or, if we take the meaning of the word common to include both the universal as contrasted with the individual experience, and the permanent, constant, or average, as distinguished from the momentary and variable percept, we may still briefly describe an illusion of perception as a deviation from the common or collective experience.

_Sources of Sense-Illusion._

Understanding sense-illusion in this way, let us glance back at the process of perception in its several stages or aspects, with the object of discovering what room occurs for illusion.

It appears at first as if the preliminary stages--the reception, discrimination, and cla.s.sification of an impression--would not offer the slightest opening for error. This part of the mechanism of perception seems to work so regularly and so smoothly that one can hardly conceive a fault in the process. Nevertheless, a little consideration will show that even here all does not go on with unerring precision.

Let us suppose that the very first step is wanting--distinct attention to an impression. It is easy to see that this will favour illusion by leading to a confusion of the impression. Thus the timid man will more readily fall into the illusion of ghost-seeing than a cool-headed observant man, because he is less attentive to the actual impression of the moment. This inattention to the sense-impression will be found to be a great co-operating factor in the production of illusions.

But if the sensation is properly attended to, can there be error through a misapprehension of what is actually in the mind at the moment? To say that there can may sound paradoxical, and yet in a sense this is demonstrable. I do not mean that there is an observant mind behind and distinct from the sensation, and failing to observe it accurately through a kind of mental short-sightedness. What I mean is that the usual psychical effect of the incoming nervous process may to some extent be counteracted by a powerful reaction of the centres. In the course of our study of illusions, we shall learn that it is possible for the quality of an impression, as, for example, of a sensation of colour, to be appreciably modified when there is a strong tendency to regard it in one particular way.

Postponing the consideration of these, we may say that certain illusions appear clearly to take their start from an error in the process of cla.s.sifying or identifying a present impression. On the physical side, we may say that the first stages of the nervous process, the due excitation of the sensory centre in accordance with the form of the incoming stimulation and the central reaction involved in the recognition of the sensation, are incomplete. These are so limited and comparatively unimportant a cla.s.s, that it will be well to dispose of them at once.

_Confusion of the Sense-Impression._

The most interesting case of such an error is where the impression is unfamiliar and novel in character. I have already remarked that in the mental life of the adult perfectly new sensations never occur. At the same time, comparatively novel impressions sometimes arise. Parts of the sensitive surface of the body which rarely undergo stimulation are sometimes acted on, and at other times they receive partially new modes of stimulation. In such cases it is plain that the process of cla.s.sing the sensation or recognizing it is not completed. It is found that whenever this happens there is a tendency to exaggerate the intensity of the sensation. The very fact of unfamiliarity seems to give to the sensation a certain exciting character. As something new and strange, it for the instant slightly agitates and discomposes the mind. Being unable to cla.s.sify it with its like, we naturally magnify its intensity, and so tend to ascribe it to a disproportionately large cause.

For instance, a light bandage worn about the body at a part usually free from pressure is liable to be conceived as a weighty ma.s.s. The odd sense of a big cavity in the mouth, which we experience just after the loss of a tooth, is probably another ill.u.s.tration of this principle. And a third example may also be supplied from the recollection of the dentist's patient, namely, the absurd imagination which he tends to form as to what is actually going on in his mouth when a tooth is being bored by a modern rotating drill. It may be found that the same principle helps to account for the exaggerated importance which we attach to the impressions of our dreams.

It is evident that all indistinct impressions are liable to be wrongly cla.s.sed. Sensations answering to a given colour or form, are, when faint, easily confused with other sensations, and so an opening occurs for illusion. Thus, the impressions received from distant objects are frequently misinterpreted, and, as we shall see by-and-by, it is in this region of hazy impression that imagination is wont to play its most startling pranks.

It is to be observed that the illusions arising from wrong cla.s.sification will be more frequent in the case of those senses where discrimination is low. Thus, it is much easier in a general way to confuse two sensations of smell than two sensations of colour. Hence the great source of such errors is to be found in that ma.s.s of obscure sensation which is connected with the organic processes, as digestion, respiration, etc., together with those varying tactual and motor feelings, which result from what is called the subjective stimulation of the tactual nerves, and from changes in the position and condition of the muscles. Lying commonly in what is known as the sub-conscious region of mind, undiscriminated, vague, and ill-defined, these sensations, when they come to be specially attended to, readily get misapprehended, and so lead to illusion, both in waking life and in sleep. I shall have occasion to ill.u.s.trate this later on.

With these sensations, the result of stimulations coming from remote parts of the organism, may be cla.s.sed the ocular impressions which we receive in indirect vision. When the eye is not fixed on an object, the impression, involving the activity of some-peripheral region of the retina, is comparatively indistinct. This will be much more the case when the object lies at a distance for which the eye is not at the time accommodated. And in these circ.u.mstances, when we happen to turn our attention to the impression, we easily misapprehend it, and so fall into illusion. Thus, it has been remarked by Sir David Brewster, in his _Letters on Natural Magic_ (letter vii.), that when looking through a window at some object beyond, we easily suppose a fly on the window-pane to be a larger object, as a bird, at a greater distance.[15]

While these cases of a confusion or a wrong cla.s.sification of the sensation are pretty well made out, there are other illusions or quasi-illusions respecting which it is doubtful whether they should be brought under this head. For example, it was found by Weber, that when the legs of a pair of compa.s.ses are at a certain small distance apart they will be felt as two by some parts of the tactual surface of the body, but only as one by other parts. How are we to regard this discrepancy? Must we say that in the latter case there are two sensations, only that, being so similar, they are confused one with another? There seems some reason for so doing, in the fact that, by a repeated exercise of attention to the experiment, they may afterwards be recognized as two.

We here come on the puzzling question, How much in the character of the sensation must be regarded as the necessary result of the particular mode of nervous stimulation at the moment, together with the laws of sensibility, and how much must be put down to the reaction of the mind in the shape of attention and discrimination? For our present purpose we may say that, whenever a deliberate effort of attention does not suffice to alter the character of a sensation, this may be pretty safely regarded as a net result of the nervous process, and any error arising may be referred to the later stages of the process of perception. Thus, for example, the taking of the two points of a pair of compa.s.ses for one, where the closest attention does not discover the error, is best regarded as arising, not from a confusion of the sense-impression, but from a wrong interpretation of a sensation, occasioned by an overlooking of the limits of local discriminative sensibility.

_Misinterpretation of the Sense-Impression._

Enough has been said, perhaps, about those errors of perception which have their root in the initial process of sensation. We may now pa.s.s to the far more important cla.s.s of illusions which are related to the later stages of perception, that is to say, the process of interpreting the sense-impression. Speaking generally, one may describe an illusion of perception as a misinterpretation. The wrong kind of interpretative mental image gets combined with the impression, or, if with Helmholtz we regard perception as a process of "unconscious inference," we may say that these illusions involve an unconscious fallacious conclusion. Or, looking at the physical side of the operation, it may be said that the central course taken by the nervous process does not correspond to the external relations of the moment.

As soon as we inspect these illusions of interpretation, we see that they fall into two divisions, according as they are connected with the process of _suggestion_, that is to say, the formation of the interpretative image so far as determined by links of a.s.sociation with the actual impression, or with an independent process of _preperception_ as explained above. Thus, for example, we fall into the illusion of hearing two voices when our shout is echoed back, just because the second auditory impression irresistibly calls up the image of a second shouter. On the other hand, a man experiences the illusion of seeing spectres of familiar objects just after exciting his imagination over a ghost-story, because the mind is strongly predisposed to frame this kind of percept. The first cla.s.s of illusions arises from without, the sense-impression being the starting-point, and the process of preperception being controlled by this. The second cla.s.s arises rather from within, from an independent or spontaneous activity of the imagination. In the one case the mind is comparatively pa.s.sive; in the other it is active, energetically reacting on the impression, and impatiently antic.i.p.ating the result of the normal process of preperception. Hence I shall, for brevity's sake, commonly speak of them as Pa.s.sive and Active Illusions.[16]

I may, perhaps, ill.u.s.trate these two cla.s.ses of illusion by the simile of an interpreter poring over an old ma.n.u.script. The first would be due to some peculiarity in the doc.u.ment misleading his judgment, the second to some caprice or preconceived notion in the interpreter's mind.

It is not difficult to define conjecturally the physiological conditions of these two large cla.s.ses of illusion. On the physical side, an illusion of sense, like a just perception, is the result of a fusion of the nervous process answering to a sensation with a nervous process answering to a mental image. In the case of pa.s.sive illusions, this fusion may be said to take place in consequence of some point of connection between the two. The existence of such a connection appears to be involved in the very fact of suggestion, and may be said to be the organic result of frequent conjunctions of the two parts of the nervous operation in our past history. In the case of active illusions, however, which spring rather from the independent energy of a particular mode of the imagination, this point of organic connection is not the only or even the main thing. In many cases, as we shall see, there is only a faint shade of resemblance between the present impression and the mental image with which it is overlaid. The illusions dependent on vivid, expectation thus answer much less to an objective conjunction of past experiences than to a capricious subjective conjunction of mental images. Here, then, the fusion of nervous processes must have another cause. And it is not difficult to a.s.sign such a cause. The antecedent activity of imagination doubtless involves as its organic result a powerful temporary disposition in the nervous structures concerned to go on acting. In other words, they remain in a state of sub-excitation, which can be raised to full excitation by a slight additional force. The more powerful this disposition in the centres involved in the act of imagination, the less the additional force of external stimulus required to excite them to full activity.

Considering the first division, pa.s.sive illusions, a little further, we shall see that they may be broken up into two sub-cla.s.ses, according to the causes of the errors. In a general way we a.s.sume that the impression always answers to some quality of the object which is perceived, and varies with this; that, for example, our sensation of colour invariably represents the quality of external colour which we attribute to the object. Or, to express it physically, we a.s.sume that the external force acting on the sense-organ invariably produces the same effect, and that the effect always varies with the external cause. But this a.s.sumption, though true in the main, is not perfectly correct. It supposes that the organic conditions are constant, and that the organic process faithfully reflects the external operation. Neither of these suppositions is strictly true. Although in general we may abstract from the organism and view the relation between the external fact and the mental impression as direct, we cannot always do so.

This being so, it is possible for errors of perception to arise through peculiarities of the nervous organization itself. Thus, as I have just observed, sensibility has its limits, and these limits are the starting-point in a certain cla.s.s of widely shared or _common_ illusions. An example of this variety is the taking of the two points of a pair of compa.s.ses for one by the hand, already referred to. Again, the condition of the nervous structures varies indefinitely, so that one and the same stimulus may, in the case of two individuals, or of the same individual at different times, produce widely unlike modes of sensation.

Such variations are clearly fitted to lead to gross _individual_ errors as to the external cause of the sensation. Of this sort is the illusory sense of temperature which we often experience through a special state of the organ employed.

While there are these errors of interpretation due to some peculiarity of the organization, there are others which involve no such peculiarity, but arise through the special character or exceptional conformation of the environment at the moment. Of this order are the illusions connected with the reflection of light and sound. We may, perhaps, distinguish the first sub-cla.s.s as organically conditioned illusions, and the second as extra-organically determined illusions. It may be added that the latter are roughly describable as common illusions. They thus answer in a measure to the first variety of organically conditioned illusions, namely, those connected with the limits of sensibility. On the other hand, the active illusions, being essentially individual or subjective, may be said to correspond to the other variety of this cla.s.s--those connected with variations of sensibility.

Our scheme of sense-illusions is now complete. First of all, we shall take up the pa.s.sive illusions, beginning with those which are conditioned by special circ.u.mstances in the organism. After that we shall ill.u.s.trate those which depend on peculiar circ.u.mstances in the environment. And finally, we shall separately consider what I have called the active illusions of sense.

It is to be observed that these illusions of perception properly so called, namely, the errors arising from a wrong interpretation of an impression, and, not from a confusion of one impression with another are chiefly ill.u.s.trated in the region of the two higher senses, sight and hearing. For it is here, as we have seen, that the interpretative imagination has most work to do in evolving complete percepts of material, tangible objects, having certain relations in s.p.a.ce, out of a limited and h.o.m.ogeneous cla.s.s of sensations, namely, those of light and colour, and of sound. As I have before observed, tactual perception, in so far as it is the recognition of an object of a certain size, hardness, and distance from our body, involves the least degree of interpretation, and so offers little room for error; it is only when tactual perception amounts to the _recognition_ of an individual object, clothed with secondary as well as primary qualities, that an opening for palpable error occurs.

With respect, however, to the first sub-cla.s.s of these illusions, namely, those arising from organic peculiarities which give a twist, so to speak, to the sensation, no very marked contrast between the different senses presents itself. So that in ill.u.s.trating this group we shall be pretty equally concerned with the various modes of perception connected with the different senses.

It may be said once for all that in thus marking off from one another certain groups of illusion, I am not unmindful of the fact that these divisions answer to no very sharp natural distinctions. In fact, it will be found that one cla.s.s gradually pa.s.ses into the other, and that the different characteristics here separated often combine in a most perplexing way. All that is claimed for this cla.s.sification is that it is a convenient mode of mapping out the subject.

CHAPTER IV.

ILLUSIONS OF PERCEPTION--_continued_.

A. _Pa.s.sive Illusions (a) as determined by the Organism._

In dealing with the illusions which are related to certain peculiarities in the nervous organism and the laws of sensibility, I shall commence with those which are connected with certain limits of sensibility.

_Limits of Sensibility._

To begin with, it is known that the sensation does not always answer to the external stimulus in its degree or intensity. Thus, a certain amount of stimulation is necessary before any sensation arises. And this will, of course, be greater when there is little or no attention directed to the impression, that is to say, no co-operating central reaction. Thus it happens that slight stimuli go overlooked, and here illusion may have its starting-point. The most familiar example of such slight errors is that of movement. When we are looking at objects, our ocular muscles are apt to execute very slight movements which escape our notice. Hence we tend, under certain circ.u.mstances, to carry over the retinal result of the movement, that is to say, the impression produced by a s.h.i.+fting of the parts of the retinal image to new nervous elements, to the object itself, and so to transform a "subjective" into an "objective" movement.

In a very interesting work on apparent or illusory movements, Professor Hoppe has fully investigated the facts of such slight movements, and endeavoured to specify their causes.[17]

Again, even when the stimulus is sufficient to produce a conscious impression, the degree of the feeling may not represent the degree of the stimulus. To take a very inconspicuous case, it is found by Fechner that a given increase of force in the stimulus produces a less amount of difference in the resulting sensations when the original stimulus is a powerful one than when it is a feeble one. It follows from this, that differences in the degree of our sensations do not exactly correspond to objective differences. For example, we tend to magnify the differences of light among objects, all of which are feebly illuminated, that is to say, to see them much more removed from one another in point of brightness than when they are more strongly illuminated. Helmholtz relates that, owing to this tendency, he has occasionally caught himself, on a dark night, entertaining the illusion that the comparatively bright objects visible in twilight were self-luminous.[18]

Again, there are limits to the conscious separation of sensations which are received together, and this fact gives rise to illusion. In general, the number of distinguishable sensations answers to the number of external causes; but this is not always the case, and here we naturally fall into the error of mistaking the number of the stimuli. Reference has already been made to this fact in connection with the question whether consciousness can be mistaken as to the character of a present feeling.

The case of confusing two impressions when the sensory fibres involved are very near one another, has already been alluded to. Both in touch and in sight we always take two or more points for one when they are only separated by an interval that falls below the limits of local discrimination. It seems to follow from this that our perception of the world as a continuum, made up of points perfectly continuous one with another may, for what we know, be illusory. Supposing the universe to consist of atoms separated by very fine intervals, then it is demonstrable that it would appear to our sensibility as a continuum, just as it does now.[19]

Two or more simultaneous sensations are indistinguishable from one another, not only when they have nearly the same local origin, but under other circ.u.mstances. The blending of partial sensations of tone in a _klang_-sensation, and the coalescence in certain cases of the impressions received by way of the two retinas, are examples of this. It is not quite certain what determines this fusion of two simultaneous feelings. It may be said generally that it is favoured by similarity between the sensations;[20] by a comparative feebleness of one of the feelings; by the fact of habitual concomitance, the two sensations occurring rarely, if ever, in isolation; and by the presence of a mental disposition to view them as answering to one external object. These considerations help us to explain the coalescence of the retinal impressions and its limits, the fusion of partial tones, and so on.[21]

It is plain that this fusion of sensations, whatever its exact conditions may be, gives rise to error or wrong interpretation of the sense-impression. Thus, to take the points of two legs of a pair of compa.s.ses for one point is clearly an illusion of perception. Here is another and less familiar example. Very cold and smooth surfaces, as those of metal, often appear to be wet. I never feel sure, after wiping the blades of my skates, that they are perfectly dry, since they always seem more or less damp to my hand. What is the reason of this? Helmholtz explains the phenomenon by saying that the feeling we call by the name of wetness is a compound sensation consisting of one of temperature and one of touch proper. These sensations occurring together so frequently, blend into one, and so we infer, according to the general instinctive tendency already noticed, that there is one specific quality answering to the feeling. And since the feeling is nearly always produced by surfaces moistened by cold liquid, we refer it to this circ.u.mstance, and speak of it as a feeling of wetness. Hence, when the particular conjunction of sensations arises apart from this external circ.u.mstance, we erroneously infer its presence.[22]

The most interesting case of illusion connected with the fusion of simultaneous sensations, is that of single vision, or the deeply organized habit of combining the sensations of what are called the corresponding points of the two retinas. This coalescence of two sensations is so far erroneous since it makes us overlook the existence of two distinct external agencies acting on different parts of the sensitive surface of the body. And this is the more striking in the case of looking at solid objects, since here it is demonstrable that the forces acting on the two retinas are not perfectly similar.

Nevertheless, such a coalescence plainly answers to the fact that these external agencies usually arise in one and the same object, and this unity of the object is, of course, the all-important thing to be sure of.

This habit may, however, beget palpable illusion in another way. In certain exceptional cases the coalescence does not take place, as when I look at a distant object and hold a pencil just before my eyes.[23] And in this case the organized tendency to take one visual impression for one object a.s.serts its force, and I tend to fall into the illusion of seeing two separate pencils. If I do not wholly lapse into the error, it is because my experience has made me vaguely aware that double images under these circ.u.mstances answer to one object, and that if there were really two pencils present I should have four visual impressions.