The errors with which we shall be concerned in this chapter are those which are commonly denoted by the term illusion, that is to say, those of sense. They are sometimes called deceptions of the senses; but this is a somewhat loose expression, suggesting that we can be deceived as to sensation itself, though, as we shall see later on, this is only true in a very restricted meaning of the phrase. To speak correctly, sense-illusions must be said to arise by a simulation of the form of just and accurate perceptions. Accordingly, we shall most frequently speak of them as illusions of perception.

In order to investigate the nature of any kind of error, it is needful to understand the kind of knowledge it imitates, and so we must begin our inquiry into the nature of illusions of sense by a brief account of the psychology of perception; and, in doing this, we shall proceed best by regarding this operation in its most complete form, namely, that of visual perception.

I may observe that in this analysis of perception I shall endeavour to keep to known facts, namely, the psychical phenomena or events which can be seen by the methods of scientific psychology to enter into the mental content called the percept. I do not now inquire whether such an analysis can help us to understand all that is meant by perception. This point will have to be touched later on. Here it is enough to say that, whatever our philosophy of perception may be, we must accept the psychological fact that the concrete mental state in the act of perception is built up out of elements, the history of which can be traced by the methods of mental science.

Psychology of Perception.

Confining ourselves for the present to the mental, as distinguished from the physical, side of the operation, we soon find that perception is not so simple a matter as it might at first seem to be. When a man on a hot day looks at a running stream and "sees" the delicious coolness, it is not difficult to show that he is really performing an act of mental synthesis, or imaginative construction. To the sense-impression[5] which his eye now gives him, he adds something which past experience has bequeathed to his mind. In perception, the material of sensation is acted on by the mind, which embodies in its present attitude all the results of its past growth. Let us look at this process of synthesis a little more closely.

When a sensation arises in the mind, it may, under certain circumstances, go unattended to. In that case there is no perception. The sensation floats in the dim outer regions of consciousness as a vague feeling, the real nature and history of which are unknown. This remark applies not only to the undefined bodily sensations that are always oscillating about the threshold of obscure consciousness, but to the higher sensations connected with the special organs of perception. The student in optics soon makes the startling discovery that his field of vision has all through his life been haunted with weird shapes which have never troubled the serenity of his mind just because they have never been distinctly attended to.

The immediate result of this process of directing the keen glance of attention to a sensation is to give it greater force and distinctness. By attending to it we discriminate it from other feelings present and past, and classify it with like sensations previously received. Thus, if I receive a visual impression of the colour orange, the first consequence of attending to it is to mark it off from other colour-impressions, including those of red and yellow. And in recognizing the peculiar quality of the impression by applying to it the term orange, I obviously connect it with other similar sensations called by the same name. If a sensation is perfectly new, there cannot, of course, be this process of classifying, and in this case the closely related operation of discriminating it from other sensations is less exactly performed. But it is hardly necessary to remark that, in the mind of the adult, under ordinary circumstances, no perfectly new sensation ever occurs.

When the sensation, or complex sensation, is thus defined and recognized, there follows the process of interpretation, by which I mean the taking up of the impression as an element into the complex mental state known as a percept. Without going into the philosophical question of what this process of synthesis exactly means, I may observe that, by common consent, it takes place to a large extent by help of a reproduction of sensations of various kinds experienced in the past. That is to say, the details in this act of combination are drawn from the store of mental recollections to which the growing mind is ever adding. In other words, the percept arises through a fusion of an actual sensation with mental representations or "images" of sensation.[6] Every element of the object that we thus take up in the act of perception, or put into the percept, as its actual size, distance, and so on, will be found to make itself known to us through mental images or revivals of past experiences, such as those we have in handling the object, moving to and from it, etc. It follows that if this is an essential ingredient in the act of perception, the process closely resembles an act of inference; and, indeed, Helmholtz distinctly calls the perception of distance an unconscious inference or a mechanically performed act of judgment.

I have hinted that these recovered sensations include the feelings we experience in connection with muscular activity, as in moving our limbs, resisting or lifting heavy bodies, and walking to a distant object. Modern psychology refers the eye's instantaneous recognition of the most important elements of an object (its essential or "primary" qualities) to a reinstatement of such simple experiences as these. It is, indeed, these reproductions which are supposed to constitute the substantial background of our percepts.

Another thing worth noting with respect to this process of filling up a sense-impression is that it draws on past sensations of the eye itself. Thus, when I look at the figure of an acquaintance from behind, my reproductive visual imagination supplies a representation of the impressions I am wont to receive when the more interesting aspect of the object, the front view, is present to my visual sense.[7]

We may distinguish between different steps in the full act of visual recognition. First of all comes the construction of a material object of a particular figure and size, and at a particular distance; that is to say, the recognition of a tangible thing having certain simple space-properties, and holding a certain relation to other objects, and more especially our own body, in space. This is the bare perception of an object, which always takes place even in the case of perfectly new objects, provided they are seen with any degree of distinctness. It is to be added that the reference of a sensation of light or colour to such an object involves the inclusion of a quality answering to the sensation, as brightness, or blue colour, in the thing thus intuited.

This part of the process of filling in, which is the most instantaneous, automatic, and unconscious, may be supposed to answer to the most constant and therefore the most deeply organized connections of experience; for, speaking generally, we never have an impression of colour, except when there are circumstances present which are fitted to yield us those simple muscular and tactual experiences through which the ideas of a particular form, size, etc., are pretty certainly obtained.

The second step in this process of presentative construction is the recognition of an object as one of a class of things, for example, oranges, having certain special qualities, as a particular taste. In this step the connections of experience are less deeply organized, and so we are able to some extent, by reflection, to recognize it as a kind of intellectual working up of the materials supplied us by the past. It is to be noted that this process of recognition involves a compound operation of classifying impressions as distinguished from that simple operation by which a single impression, such as a particular colour, is known. Thus the recognition of such an object as an orange takes place by a rapid classing of a multitude of passive sensations of colour, light, and shade, and those active or muscular sensations which are supposed to enter into the visual perception of form.

A still less automatic step in the process of visual recognition is that of identifying individual objects, as Westminster Abbey, or a friend, John Smith. The amount of experience that is here reproduced may be very large, as in the case of recognizing a person with whom we have had a long and intimate acquaintance.

If the recognition of an object as one of a class, for example, an orange, involves a compound process of classing impressions, that of an individual object involves a still more complicated process. The identification of a friend, simple as this operation may at first appear, really takes place by a rapid classing of all the salient characteristic features which serve as the visible marks of that particular person.

It is to be noted that each kind of recognition, specific and individual, takes place by a consciousness of likeness amid unlikeness. It is obvious that a new individual object has characters not shared in by other objects previously inspected. Thus, we at once class a man with a dark-brown skin, wearing a particular garb, as a Hindoo, though he may differ in a host of particulars from the other Hindoos that we have observed. In thus instantly recognizing him as a Hindoo, we must, it is plain, attend to the points of similarity, and overlook for the instant the points of dissimilarity. In the case of individual identification, the same thing happens. Strictly speaking, no object ever appears exactly the same to us on two occasions. Apart from changes in the object itself, especially in the case of living beings, there are varying effects of illumination, of position in relation to the eye, of distance, and so on, which very distinctly affect the visual impression at different times. Yet the fact of our instantly recognizing a familiar object in spite of these fluctuations of appearance, proves that we are able to overlook a very considerable amount of diversity when a certain amount of likeness is present.

It is further to be observed that in these last stages of perception we approach the boundary line between perception and inference. To recognize an object as one of a class is often a matter of conscious reflection and judgment, even when the class is constituted by obvious material qualities which the senses may be supposed to apprehend immediately. Still more clearly does perception pass into inference when the class is constituted by less obvious qualities, which require a careful and prolonged process of recollection, discrimination, and comparison, for their recognition. Thus, to recognize a man by certain marks of gesture and manner as a military man or a Frenchman, though popularly called a perception, is much more of an unfolded process of conscious inference. And what applies to specific recognition applies still more forcibly to individual recognition, which is often a matter of very delicate conscious comparison and judgment. To say where the line should be drawn here between perception and observation on the one hand, and inference on the other, is clearly impossible. Our whole study of the illusions of perception will serve to show that the one shades off into the other too gradually to allow of our drawing a hard and fast line between them.

Finally, it is to be noted that these last stages of perception bring us near the boundary line which separates objective experience as common and universal, and subjective or variable experience as confined to one or to a few. In the bringing of the object under a certain class of objects there is clearly room for greater variety of individual perception. For example, the ability to recognize a man as a Frenchman turns on a special kind of previous experience. And this transition from the common or universal to the individual experience is seen yet more plainly in the case of individual recognition. To identify an object, say a particular person, commonly presupposes some previous experience or knowledge of this object, and the existence in the past of some special relation of the recognizer to the recognized, if only that of an observer. In fact, it is evident that in this mode of recognition we have the transition from common perception to individual recollection.[8]

While we may thus distinguish different steps in the process of visual recognition, we may make a further distinction, marking off a passive and an active stage in the process. The one may be called the stage of preperception, the other that of perception proper.[9] In the first the mind holds itself in a passive attitude, except in so far as the energies of external attention are involved. The impression here awakens the mental images which answer to past experiences according to the well-known laws of association. The interpretative image which is to transform the impression into a percept is now being formed by a mere process of suggestion.

When the image is thus formed, the mind may be said to enter upon a more active stage, in which it now views the impression through the image, or applies this as a kind of mould or framework to the impression. This appears to involve an intensification of the mental image, transforming it from a representative to a presentative mental state, making it approximate somewhat to the full intensity of the sensation. In many of our instantaneous perceptions these two stages are indistinguishable to consciousness. Thus, in most cases, the recognition of size, distance, etc., takes place so rapidly that it is impossible to detect the two phases here separated. But in the classification of an object, or the identification of an individual thing, there is often an appreciable interval between the first reception of the impression and the final stage of complete recognition. And here it is easy to distinguish the two stages of preperception and perception. The interpretative image is slowly built up by the operation of suggestion, at the close of which the impression is suddenly illumined as by a flash of light, and takes a definite, precise shape.

Now, it is to be noted that the process of preperception will be greatly aided by any circumstance that facilitates the construction of the particular interpretative image required. Thus, the more frequently a similar process of perception has been performed in the past, the more ready will the mind be to fall into the particular way of interpreting the impression. As G.H. Lewes well remarks, "The artist sees details where to other eyes there is a vague or confused mass; the naturalist sees an animal where the ordinary eye only sees a form." This is but one illustration of the seemingly universal mental law, that what is repeatedly done will be done more and more easily.

The process of preperception may be shortened, not only by means of a permanent disposition to frame the required interpretative scheme, the residuum of past like processes, but also by means of any temporary disposition pointing in the same direction. If, for example, the mind of a naturalist has just been occupied about a certain class of bird, that is to say, if he has been dwelling on the mental image of this bird, he will recognize one at a distance more quickly than he would otherwise have done. Such a simple mental operation as the recognition of one of the less common flowers, say a particular orchid, will vary in duration according as we have or have not been recently forming an image of this flower. The obvious explanation of this is that the mental image of an object bears a very close resemblance to the corresponding percept, differing from it, indeed, in degree only, that is to say, through the fact that it involves no actual sensation. Here again we see illustrated a general psychological law, namely, that what the mind has recently done, it tends (within certain limits) to go on doing.

It is to be noticed, further, that the perception of a single object or event is rarely an isolated act of the mind. We recognize and understand the things that surround us through their relations one to another. Sometimes the adjacent circumstances and events suggest a definite expectation of the new impression. Thus, for example, the sound of a gun heard during a walk in the country is instantly interpreted by help of suggestions due to the previous appearance of the sportsman, and the act of raising the gun to his shoulder. It may be added that the verbal suggestions of others act very much like the suggestions of external circumstances. If I am told that a gun is going to be fired, my mind is prepared for it just as though I saw the sportsman.[10]

More frequently the effect of such surrounding circumstances is to give an air of familiarity to the new impression, to shorten the interval in which the required interpretative image is forthcoming. Thus, when travelling in Italy, the visual impression answering to a ruined temple or a bareheaded friar is construed much more rapidly than it would be elsewhere, because of the attitude of mind due to the surrounding circumstances. In all such cases the process of preperception connected with a given impression is effected more or less completely by the suggestions of other and related impressions.

It follows from all that has been just said that our minds are never in exactly the same state of readiness with respect to a particular process of perceptional interpretation. Sometimes the meaning of an impression flashes on us at once, and the stage of preperception becomes evanescent. At other times the same impression will fail for an appreciable interval to divulge its meaning. These differences are, no doubt, due in part to variations in the state of attention at the moment; but they depend as well on fluctuations in the degree of the mind's readiness to look at the impression in the required way.

In order to complete this slight analysis of perception, we must look for a moment at its physical side, that is to say, at the nervous actions which are known or supposed with some degree of probability to accompany it.

The production of the sensation is known to depend on a certain external process, namely, the action of some stimulus, as light, on the sense-organ, which stimulus has its point of departure in the object, such as it is conceived by physical science. The sensation arises when the nervous process is transmitted through the nerves to the conscious centre, often spoken of as the sensorium, the exact seat of which is still a matter of some debate.

The intensification of the sensation by the reaction of attention is supposed to depend on some reinforcement of the nervous excitation in the sensory centre proceeding from the motor regions, which are hypothetically regarded as the centre of attention.[11] The classification of the impression, again, is pretty certainly correlated with the physical fact that the central excitation calls into activity elements which have already been excited in the same way.

The nervous counterpart of the final stage of perception, the synthesis of the sensation and the mental representation, is not clearly ascertained. A sensation clearly resembles a mental image in quality. It is most obviously marked off from the image by its greater vividness or intensity. Agreeably to this view, it is now held by a number of eminent physiologists and psychologists that the nervous process underlying a sensation occupies the same central region as that which underlies the corresponding image. According to this theory, the two processes differ in their degree of energy only, this difference being connected with the fact that the former involves, while the latter does not involve, the peripheral region of the nervous system. Accepting this view as on the whole well founded, I shall speak of an ideational, or rather an imaginational; and a sensational nervous process, and not of an ideational and a sensational centre.[12]

The special force that belongs to the representative element in a percept, as compared with that of a pure "perceptional" image,[13] is probably connected with the fact that, in the case of actual perception, the nervous process underlying the act of imaginative construction is organically united to the initial sensational process, of which indeed it may be regarded as a continuation.

For the physical counterpart of the two stages in the interpretative part of perception, distinguished as the passive stage of preperception, and the active stage of perception proper, we may, in the absence of certain knowledge, fall back on the hypothesis put forward by Dr. J. Hughlings Jackson, in the articles in Brain already referred to, namely, that the former answers to an action of the right hemisphere of the brain, the latter to a subsequent action of the left hemisphere. The expediting of the process of preperception in those cases where it has frequently been performed before, is clearly an illustration of the organic law that every function is improved by exercise. And the temporary disposition to perform the process due to recent imaginative activity, is explained at once on the physical side by the supposition that an actual perception and a perceptional image involve the activity of the same nervous tracts. For, assuming this to be the case, it follows, from a well-known organic law, that a recent excitation would leave a temporary disposition in these particular structures to resume that particular mode of activity.

What has here been said about visual perception will apply, mutatis mutandis, to other kinds. Although the eye is the organ of perception par excellence, our other senses are also avenues by which we intuit and recognize objects. Thus touch, especially when it is finely developed as it is in the blind, gives an immediate knowledge of objects—a more immediate knowledge, indeed, of their fundamental properties than sight. What makes the eye so vastly superior to the organ of touch as an instrument of perception, is first of all the range of its action, taking in simultaneously a large number of impressions from objects at a distance as well as near; and secondly, though this may seem paradoxical, the fact that it gives us so much indirectly, that is, by way of association and suggestion. This is the interesting side of visual perception, that, owing to the vast complex of distinguishable sensations of light and colour of various qualities and intensities, together with the muscular sensations attending the varying positions of the organ, the eye is able to recognize at any instant a whole external world with its fundamental properties and relations. The ear comes next to the eye in this respect, but only after a long interval, since its sensations (even in the case of musical combinations) do not simultaneously order themselves in an indefinitely large group of distinguishable elements, and since even the comparatively few sensations which it is capable of simultaneously receiving, being altogether passive—that is to say, having no muscular accompaniments—impart but little and vague information respecting the external order. It is plain, then, that in the study of illusion, where the indirectly known elements are the thing to be considered, the eye, and after this the ear, will mostly engage our attention.[14]So much it seemed needful to say about the mechanism of perception, in order to understand the slight disturbances of this mechanism that manifest themselves in sense-illusion. It may be added that our study of these illusions will help still further to elucidate the exact nature of perception. Normal mental life, as a whole, at once illustrates, and is illustrated by, abnormal. And while we need a rough provisional theory of accurate perception in order to explain illusory perception at all, the investigation of this latter cannot fail to verify and even render more complete the theory which it thus temporarily adopts.

Illusions of Perception.

With this brief psychological analysis of perception to help us, let us now pass to the consideration of the errors incident to the process, with a view to classify them according to their psychological nature and origin.

And here there naturally arises the question, How shall we define an illusion of perception? When trying to fix the definition of illusion in general, I practically disposed of this question. Nevertheless, as the point appears to me to be of some importance, I shall reproduce and expand one or two of the considerations then brought forward.

It is said by certain, philosophers that perception, as a whole, is an illusion, inasmuch as it involves the fiction of a real thing independent of mind, yet somehow present to it in the act of sense-perception. But this is a question for philosophy, not for science. Science, including psychology, assumes that in perception there is something real, without inquiring what it may consist of, or what its meaning may be. And though in the foregoing analysis of perception, viewed as a complex mental phenomenon or psychical process, I have argued that a percept gets its concrete filling up out of elements of conscious experience or sensations, I have been careful not to contend that the particular elements of feeling thus represented are the object of perception or the thing perceived. It may be that what we mean by a single object with its assemblage of qualities is much more than any number of such sensations; and it must be confessed that, on the face of it, it seems to be much more. And however this be, the question, What is meant by object; and is the common persuasion of the existence of such an entity in the act of perception accurate or illusory? must be handed over to philosophy.

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 circumstances 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 circumstances 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 classification 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 classifying 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 class, 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 classing 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 classify 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 mass. The odd sense of a big cavity in the mouth, which we experience just after the loss of a tooth, is probably another illustration 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 classed. 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 classification 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 mass 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 illustrate this later on.

With these sensations, the result of stimulations coming from remote parts of the organism, may be classed 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 circumstances, 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 classification 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 compasses 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 compasses 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 pass to the far more important class 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 association 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 class of illusions arises from without, the sense-impression being the starting-point, and the process of preperception being controlled by this. The second class arises rather from within, from an independent or spontaneous activity of the imagination. In the one case the mind is comparatively passive; in the other it is active, energetically reacting on the impression, and impatiently anticipating the result of the normal process of preperception. Hence I shall, for brevity's sake, commonly speak of them as Passive and Active Illusions.[16]

I may, perhaps, illustrate these two classes of illusion by the simile of an interpreter poring over an old manuscript. The first would be due to some peculiarity in the document 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 classes 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 passive 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 assign 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, passive illusions, a little further, we shall see that they may be broken up into two sub-classes, according to the causes of the errors. In a general way we assume 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 assume 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 assumption, 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 class of widely shared or common illusions. An example of this variety is the taking of the two points of a pair of compasses 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-class 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 class—those connected with variations of sensibility.

Our scheme of sense-illusions is now complete. First of all, we shall take up the passive illusions, beginning with those which are conditioned by special circumstances in the organism. After that we shall illustrate those which depend on peculiar circumstances 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 illustrated 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 space, out of a limited and homogeneous class 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-class 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 illustrating 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 class gradually passes into the other, and that the different characteristics here separated often combine in a most perplexing way. All that is claimed for this classification is that it is a convenient mode of mapping out the subject.