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IN POPULAR parlance "illusion" is a very comprehensive term. It is almost synonymous with that of
"error." Sully remarks that with many it suggests even insanity. But this for the psychologist is quite as much an "illusion" as any error of perception. In looser expression it may do good service as a name for various errors of perception and judgment, but it should never be mistaken for those organic and fixed disturbances which are implied by insanity and persistent hallucinations. It more generally imports those temporary variations from the normal standard of perception that induce us to disregard what we call illusions in our adaptive life. In the present discussion of them, therefore, we must give illusion a sufficiently definite meaning to distinguish it, on the one hand, from normal mental operations and on the other from hallucination, and perhaps also from the graver mental disturbances involved in pathology. It is also distinguishable from fallacy, which is an error in reasoning.
Illusion is usually defined as an error of perception, and, if too narrow limits are not assigned to
"perception," there can be no objection to this conception of it. But often, owing to certain technical limitations assignable to perception and to the interposition of judgment in the phenomena, illusion is sometimes regarded as an error of judgment. This conception presumably distinguishes it from fallacy, which, as just remarked, is an error of reasoning. There are certain errors of judgment which either participate in illusion or constitute it, and whether it is limited to this or not will depend upon the place assigned to mental phenomena often ascribed to perception. No doubt it is hard to fix the limits between perception and judgment, as both are so organically related to the most fundamental of our elementary states of knowledge, and psychologists have varied so much in the exact functions to be named by perception that they give correspondingly elastic conception to the phenomena of illusion. Perhaps in the distinction from hallucination, which is an organic disturbance, we have the best limitation of illusion, though it is often hard in concrete cases to distinguish between them. In type, however, they are easily enough distinguishable, as hallucinations have a fixity in most cases that prevents any correction of their occurrence, while illusions are usually corrected very easily. Hallucinations are more or less permanent aberrations of function; illusions are more or less temporary aberrations of function, and usually not the same functions exactly that are involved in the former, though they interpenetrate. Illusion may then be regarded as comprehending errors of perception and judgment which are more closely related to the normal actions of the mind than are hallucinations.
Sully's definition is one of the best. He defines illusion provisionally "as any species of error which counterfeits the form of immediate, self-evident, or intuitive knowledge, whether as sense-perception or otherwise." This distinguishes it from normal mental action, but does not make the distinction from hallucination apparent. To me illusion lies between the normal and hallucinatory perception, and is distinguished from both of them; from the first in being an error and from the second in being less fixed and organic. I should emphasize the inclusion of judgment in the phenomena, and perhaps lay the most blame upon it for the error, while in hallucination I should attribute the primary cause to abnormal sensory functions. Possibly we might say that the primary distinction between illusion and hallucination would be just this: that in illusion the primary source of error is mistaken judgment, and in hallucination the primary source is abnormal sensory action more or less organically aberrant. They will, of course, often shade into each other, and hence I am here but distinguishing the types, a distinction which can be made more clear by illustration.
As a clear illustration of illusions I may give the following in my own experience. When a boy I was riding early in the morning to the Ohio State fair. As we had to ride some twenty miles, we started about three o'clock in the morning, and I had awakened from a sleep after riding some seven miles. It was very early dawn, and, on looking out of the carriage through the woods, I saw an immense palace of Grecian architecture. I was on the point of remarking to my father that I did not know there was such a palace in this locality, when I noticed it changing its form. In a moment, and before
I could speak of it, the palace vanished into an open field beyond the woods. The trees and skies had suggested the palace, and the motion of the carriage interrupted the illusion.
Again, after lecturing to my class at Columbia University on the subject of space-perception, I was walking down Madison Avenue, on which there are no trees whatever. But at a certain point I noticed ahead of me both sides of the avenue lined with trees. Astonished at the vision, I stopped to see what it meant, and saw some distance in front of me a moving van with a picture of a street in a city lined with trees on both sides, and this had fitted exactly into the perspective of Madison Avenue. The illusion was of course quickly corrected.
The illusion in these cases consists in the existence of a sense-perception more or less suggestive of the thing apparently seen, and the state of mind being favorable to seeing that particular thing, the sensation or impression is correspondingly distorted, and an object is apparently seen which is not there. Moreover, the illusion is characterized by an impression or stimulus in the sense which does the apparent perceiving, and the whole effect is quickly corrected, as it is not due to organic disturbance in the sensory centres, but rather to temporary preoccupation of the interpreting functions in a way to distort the sense-perception.
An illustration of an hallucination is the following. A certain gentleman has only to throw his head back upon his collar, when the pressure of the collar on a blood-vessel in the neck gives rise to the appearance of a human hand moving down from above his head before his face. To stop it the man has only to put his head in its normal position and remove the pressure of the collar on his neck. Here we have a tactual stimulus and a visual appearance, and hence a phenomenon that cannot be technically called an illusion, as it does not represent a distorted sense-impression within the sense having the perception. This is not always the characteristic of an hallucination, but when it does occur it best represents the functional action involved in hallucination, and such action is called secondary stimulus, because it involves stimulation in one sense and reaction in another, and is not properly an interpretation or misinterpretation of a proper stimulus.
In another case a physician can see an apparition of his deceased son in the left of the field of vision whenever he turns his attention to it or thinks of it. Nothing is apparently said in the case, and the apparition moves with the motion of the eyes. That is, the effort to focus on the apparition avails to cause it to move, showing that some organic disturbance, perhaps either in the retina or brain-centre gives rise, with expectancy, to the apparition, which seems persistent.
In these illustrations the primary factor is not misinterpretation of sensory stimuli, but abnormal stimuli, and where they are secondary they exhibit distorted central action of a sensory character. Illusions are perhaps either primarily misinterpretations of impressions or these impressions are more nearly like the normal. But hallucinations persist more fixedly as simulations of external reality, and are corrected with much more difficulty, if they can be corrected at all.
These illustrations suffice to indicate the distinction between illusions and hallucinations for general purposes. I do not pretend that they are accurate and complete accounts of either their nature or their differences, but only that the criteria provided suffice for all practical purposes in the examination of problems in psychic research. As
I have already remarked, illusions and hallucinations shade into each other in certain concrete instances, but in their types or most frequent manifestation illusions are the primary result of misinterpretation of a normal stimulus, while hallucination is primarily due to organic sensory defects, whether central or peripheral. Organic intellectual disturbances are sometimes called hallucinations, but I think it better to call them
delusions. Of this again. All that I want to emphasize at present is the sensory character of the true hallucination, which persists in its simulation of reality more than do illusions. Misinterpretation is as important a factor of illusion as aberrant sensory action.
We can perhaps best understand illusions, however, by dividing them into their various types, according to the predominance of the factor which determines their nature. In a general division or classification of illusions, however, I wish to remark a distinction which will be of some importance in the treatment and discussion of problems in psychic research. This distinction relates to those illusions which characterize all normal perception and represent organic conditions of the sensorium, while another class represent the influence of the mental state on the sensory impression to distort it, or misinterpret its meaning. In pursuance of the idea expressed in this, I think it may serve a useful end to distinguish illusions by their relation to the organism and to its functions. I shall therefore divide them into two general types, with such subdivisions as we may please to make or discover. These two types I shall call
Organic and Functional Illusions. Both are associated with sensory irregularities. Organic illusions are those which represent an abnormal relation between stimulus and sensory reaction, and so may as regularly characterize sense-perception as normal activity. They therefore occur according to certain definite laws of the organism, and hence are not sporadic or occasional phenomena, but are quite as normal in respect of their occurrence under their specified conditions as are normal perceptions. Functional illusions are those which represent an abnormal influence of interpretation or mental functions on the sensory impression. The physiological facts are just what they are in normal perception, but some distortion of interpreting functions avails to distort the apparent object into something else than what it really is. We shall proceed to illustrate and explain both types of illusion, and shall recognize at the same time that there may be forms of such illusions that interpenetrate or overlap both these
types.
Organic
Illusions
Perhaps the best illustration of organic illusions is the phenomenon of color contrast. If a piece of
gray paper be laid upon a patch of bright blue, and both covered with a piece of tissue-paper quite translucent, the gray will appear to be yellow. If the background on which the gray is placed be yellow, the gray will appear blue. If the background be red, the gray will appear green, and if the background be green, the gray will appear red. Whatever the cause of this contrast, or perception of the complementary color, there is a phenomenon which appears to violate the well-known physiological and chemical explanation of color-perception. We seem to see colors that are not in fact presented on the retina. According to the normal organic laws of optics, we ought to see the colors as they are presented. But under these peculiar conditions we see a color that is the complementary of the background, and the judgment is an illusion. This illusion is organic because it is the uniform experience of vision in practically all people, and is as fixed and regular as normal perception itself. Only the conditions of the stimulus are abnormal, or irregular.
The various illusions produced by mathematical perspective in imitation of solid objects illustrate the same kind of illusion. The geometrical figure of a cube can be seen in either of two positions, or to represent a cube in either of two positions. It is the same with figures representing a screen or a tube. Take also the geometrical representation of a stairway which can be seen at will either from the upper or lower side; in one as if for ascent and in the other as if standing under it.
Stereoscopic pictures and figures represent the same phenomenon. They are drawn so as to represent the binocular parallax, which is always an important feature in normal vision, and the consequence is that, with the stereoscope, they appear to represent clearly solid objects or true perspective. This parallax of which I speak is constituted in normal vision by the slight difference between the retinal images produced by solid objects. The effect in the visual process is to bring out more clearly the perception of solidity, or the third dimension. If we imitate this parallax or disparateness of retinal images, as we can in geometrical figures, we elicit this visual process so as to produce the illusion of solidity where it does not exist. This imitation is what is effected in stereoscopic pictures. They are made with a slight difference in their representation of the object, so that the retinal images are not exactly alike. The effect is apparent solidity as in real objects. The interesting feature of the fact also is that the solidity or perspective is as clear and stable as in the perception of real objects. We should not be aware of any illusion in the phenomena but for our consciousness that no such real objects are present as appear to be. If we could divest ourselves of the consciousness that surrounding objects of a different kind and unrelated to the stereoscopic pictures were not present, we should not be able to discover our illusion at all. The apparent reality of what we see in such cases is so distinct that it requires a special knowledge of the conditions under which the phenomena occur to even ascertain their illusory character. The organic functions of vision act normally, and the phenomena are not ordinarily interpretative, though that function is admitted into the effect. But the stimulus or sense-impression is modified so as to take on the character of the stimulus of the real solid object, and the mind has no alternative to the judgment which it forms. The illusion is an organic one, because it represents the normal action of the sensory process and is characteristic of all persons.
The phenomena of mathematical perspective and light and shade illustrate the same general process. In real objects the apparent size diminishes with the distance of the objects from us. The intensity of light also decreases in the same way, and shadows are indications of space-relations and with mathematical perspective may be used to affect the perception of distance. If, then, we draw geometrical figures in such a way as to imitate the retinal images of solid objects in the characteristics named, we should expect to elicit the natural perception of distance and solidity. This is exactly what takes place. If we draw two lines so that they are not exactly parallel, but approaching each other slightly, they may be seen as a railway track. This will be much clearer if we have other appropriate objects drawn in the same field. The representation of a cube, mentioned above, illustrates the same fact also.
Aerial perspective, as it is sometimes called, also produces the effect of modifying our perceptions. It is the effect of the atmosphere on the judgment of apparent distance. When the air is misty or smoky it makes objects appear more distant. When it is clear they seem nearer. The effect is due to the association of distinctness and indistinctness with the actual and known distance of objects. In normal vision distant objects are less distinct than nearer objects, and when any condition of the atmosphere reproduces an unnatural distinctness or indistinctness, the associated judgment of distance is suggested.
In mathematical and aerial perspective, however, interpreting functions enter very largely into the perceptions. The organic functions are perhaps less dominant than in binocular perception, but they are apparently active, though fused with inference and association to such an extent that it is difficult to recognize the organic and functional influences. These seem to be present from the uniform and fixed habits of normal perception in such circumstances.
After-images are a good type of organic illusion. If we look at the sun directly for a few seconds, and then look at the sky at some other point, we can see an apparition or image of the sun, usually in the complementary color. This apparent perception of it may last some time before fading away into a mere shadow. If we look at a bright light, say an incandescent electric light or any very bright light of the kind, and then look at the wall or some appropriate background, we are likely to see a reproduction of the light on this background, and it is usually in some complementary color. This is what is called an after-image, and it represents all the appearance of an external reality like the original object or light. But for the circumstances with which we are usually familiar the apparition might be taken for a real object. I have been able, in looking through a window at a landscape or streets of a city, to reproduce in an after-image, by closing my eyes, the exact view at which I was looking, with its color, perspective, and all. This exact reproduction of the visual impression as an apparent object is called the positive after-image, while the appearance of the outline or same image in the complementary color is called the negative after-image. In both there is a retinal reaction, the positive image representing the exact sensory reaction of a real sensory object or reality. The phenomenon might be called an hallucination but for its transient character. It is, however, organic in any case, and represents erroneous perception in its maladjustment of sensory function.
Another type of illusion illustrates organic influences. I refer to the apparent motion of objects when it is we ourselves that are in motion. Those who do not feel their own motion or are not conscious of it in some way -and this is especially true of children at first - when in a train of moving cars, will see the landscape apparently travelling in the opposite direction. It often takes time and effort to correct this impression. The same illusion in a modified form occurs with nearly all people when waiting for their train to start. They often think it has started, only to find that it is a train or car opposite that is moving in the opposite direction. This illusion is so strong with myself that, when it occurs, unless I can look at some stationary object, it is almost impossible to correct it. In the former instances, those of the apparently moving landscape, the cause is the real motion of the retinal image not corrected by the consciousness of the bodily motion in space. I have seen this phenomenon illustrated by the appearance of the gaslight moving across the room, caused by the actual motion of the eyes into a parallel position as sleep approached, and without the consciousness that the eyes were so moving. The retinal image of the light moved across the retina and produced the illusion of actual motion in the light. In the case of the apparent motion of a car opposite the observer, we have retinal motion of the image, but it is accompanied by a tactual illusion of real motion of the car in which we sit. We can correct it only by visual comparison of the known impression with other objects in the field that remain stable. The tactual illusion or hallucination, so to speak, is arrested. In all of them, however, organic influences operate, whatever the interpretative functions, and these are factors undoubtedly. But the organic reactions of the sensorium are so natural a process of the effect that they may be regarded as the dominant influence.
The localization of sensations in amputated parts of the limbs is another illustration of organic illusions. Some question may arise as to the nature of this phenomenon, but it undoubtedly represents a judgment of an existing object or limb that is not the fact. The explanation of it is not the point of interest at present, but merely the fact that sensations are assigned a locality which is physically impossible under the circumstances.
Narcotics and poisons often affect the sensory organism so as to give rise to abnormal perceptions, which are illusory in comparison with what is accepted as normal. Certain poisons affect color perceptions, as santonin, according to Sully, makes colorless objects look
yellow.
Functional
Illusions
I have explained that functional illusions represent an abnormal influence of the interpreting acts of the mind, or inference and association, in distorting what we should most naturally take for something else than the apparent perception. In this conception of them, however, I recognize that the distinction between them and organic illusions will not always be clear. They will often overlap each other, and functional illusions will be most distinct in those instances in which impressions are greatly distorted, owing to subjective states of mind. They will often merge even into fallacies of reasoning. But those which are more closely allied to errors in perception will have the characteristic of a misperceived object.
Mathematical figures representing solid objects or perspective illustrate this inferential function to some extent, though they ally their illusions to the organic type. The organic element is indicated in certain fixed organic conditions in the impression which limit the inferences which we might draw from their appearances. But inference and association operate in them to a sufficient degree to admit them at the same time to a place among the functional illusions caused in this way. Aerial perspective and intervening objects also illustrate the same phenomena. From them we infer perhaps more than we see, but owing to the peculiar nature of perception we seem actually to see what is in fact the product of memory and inference.
An illustration of functional illusion bordering on the organic is one which may represent a frequent type. There was a picture of a flower in my room which, when seen at the proper distance, appeared to represent a little, queer old man doubled up in a funny position. The first time I saw this picture I did not recognize the flower, but thought I saw this funny old man. I approached the picture to see it more distinctly and found that it was a flower. I returned to my original position, and the little old man reappeared in place of the flower, and never afterward could I look at that picture at this distance without seeing this queer old man, though I knew well enough that it was a flower. The preconception established by the first experience was strong enough to prevent the corrected judgment from being more than an inner judgment, not a perception. The illusion always remained. Recently I had a similar experience with the reflection of a window and some candlesticks on a mirror in a photograph. The appearance at a certain distance was of a peculiar old man with a very high skullcap on his head. Close inspection corrected the illusion, but it would reappear when I resumed the distance at which I first saw the photograph. The general resemblance in the pictures to the objects apparently seen had sufficed to distort the impression, and this experience was sufficient to keep up the illusion after it was once created.
The primary influence in producing the illusions in these and similar instances is indistinctness of certain parts of the retinal image. The evidence of this is the fact that the illusion disappears when the object or picture is viewed at close range. What the eye seized was those characteristics which it sees most clearly, and the mind interprets the impression in accordance with past experience. In the instances mentioned the most distinct features of the object were comparatively clear, and others were not clear enough to suggest their part in the impression. The consequence was that the mind would take account of what it was most aware of, and perhaps its memory and imagination would unconsciously introduce elements from the past and from constructive tendencies of the mind into the product. But leaving the subjective and mental influences on what we see out of account, the main cause externally of the illusion is indistinctness of the impression as affected by the relation of the object to sense. The causes of this indistinctness may be various. Sometimes it may be distance, sometimes it may be peculiarities in light and shade in the object, and sometimes it may be the dimness of the light in which the object exists. We can hardly lay down any special law for all cases, but the most general one, and this will be any influence which dims the retinal image.
General illustrations with which we are all familiar are found in the phenomena of seeing forms in the clouds, distorting objects in the dark, perceiving animal or human forms in physical objects, as the "Old Man of the Mountain." These occur everywhere and at all times, and readers will recall them without multiplying instances. It suffices to emphasize the cause of them as something to consider when we come to discuss phenomena purporting to represent agencies beyond sense-experience.
We do not always, if ever, seriously think of it, but pictures are one of the best illustrations of illusion that can be given. They are combinations of light and shade with mathematical perspective so as to represent real objects. A good artist can so imitate reality as to produce what we call the illusion of it, that is, so distinct an appearance of real objects with their solidity as to be taken for them. The legend of Apelles, or some Greek artist, illustrates this. It was said of him that he painted fruit so well that the birds came and tried to peck it. Landscape views illustrate reality so perfectly that one can easily lose himself in the feeling that he is looking at actual scenes. This is quite noticeable in good theatrical scenery when the light is properly managed, though, if close to it, the view would present no illusion at all. Size, indistinctness of form and color, and various devices in imitation of the influences which nature uses to suggest distance and perspective are the means of producing these illusions in artificial representations. The photograph does it to perfection, though it relies upon fewer agencies than are found in reality. Light and shade are its only resource.
One very interesting instance of illusion in pictures is that with which we are all familiar, namely, the apparent change of position in objects when the spectator changes his position. If we look at the picture of a person from either side and then change our position to the opposite side, the person will have appeared to have changed his position. If the picture be that of a profile this illusion is much more apparent, but is equally an illusion in all other cases. If we watch carefully while we change our position, we shall appear to see the person actually turning his face toward us. The cause of this is the simple fact that, in plain pictures, which have no actual solidity in their forms, the view is the same for the observer in all positions, and as the view is not the same for stationary solid objects, we naturally see pictures as if the object had changed, as this change in real objects must occur if their impressions remain the same when the observer changes his position. In viewing solid objects, a change of position by the spectator is not followed by exactly the same retinal images as in pictures, and hence the judgment must be different. In pictures the illusion is due to the identity of retinal images in situations which normal experience represents as different, and hence our judgment sees the phenomena from the standpoint of normal experience when asking for the appearance of the picture as compared with the past, which is the standard of judgment.
Another and equally interesting illusion is the following: If we look at a windmill wheel, such as is used in wind-pumps, while it is revolving in a position oblique to the observer, we may not be able to tell in which direction it is revolving. This depends upon the question whether the oblique direction of the wheel's axis is apparently on our left or our right. The retinal impression or image is the same for both positions, and if binocular influences are either too indistinct or imperceptible we are left only to geometrical considerations in the formation of our judgments. We may thus apparently see the wheel in either of two positions, and its motion will appear to accord with this apparent position, now seeming to be in the direction of left to right and again from right to left, and in either case completely the opposite of what it appears to be in the alternative direction. The phenomenon associates organic with functional influences.
There is a large class of illusions in which the primary factor in their production is the state of mind in the observer. I recall one instance in my own experience. I had called the roll of my class, and a certain young man by the name of Macaulay was absent, but came in before the end of the hour. He called my attention to the fact at the close of the lecture, and as I was in a hurry to meet another class I waited until I arrived in another room to mark his attendance. When I sat down I noticed a piece of paper on the desk in front of me and underscored, as I thought, was the name Macaulay. I was struck with the coincidence, and in looking at the word found it was
manager. Here the mental interest in not forgetting to note the presence of a man whom I had marked as absent had the effect of distorting the sense-impression and of making it appear quite different from what it actually was.
Prof. James narrates a similar personal experience. "I remember one night," he says, "in Boston, whilst waiting for a 'Mount Auburn' car to bring me to Cambridge, reading most distinctly that name on the sign-board of a car, on which, as I afterward learned, 'North Avenue' was painted. The illusion was so vivid that I could hardly believe my eyes had deceived me." This Prof. James classifies under "proof-readers' illusions," and I may remark that my own absorption in the thought of what I write makes it exceedingly difficult for me to detect errors in print. I often see a word rightly spelled when it is in fact wrongly spelled.
"The whole past mental life," says Sully, "with its particular shape of experience, its ruling emotions, and its habitual direction of fancy, serves to give a particular color to new impressions, and so to favor illusion. There is a 'personal equation' in perception as in belief, - an amount of erroneous deviation from the common average view of external things, which is the outcome of individual temperament and habits of mind. Thus a naturally timid man will be in general disposed to see ugly and fearful objects, where a perfectly unbiased mind perceives nothing of the kind; and the forms which these objects of dread will assume are determined by the character of his past experience, and by the customary direction of his imagination."
Such phenomena could be illustrated at much greater length, but sufficient instances have been given to explain the liability of the mind to mistaken judgments in certain normal perceptions. In discussing normal sense-perception
I remarked the difficulty of assuring ourselves of an infallible criterion for external reality, and this question is again suggested by the phenomena of illusion. But with the fact that illusion does not affect the existence of external reality, but only the nature of it, we may remark that the skeptical limitations which it assigns to our perceptions relate to the correctness of our conceptions and judgments regarding the totality of this external object. The maladjustment between sensation or impression and the interpreting function of the mind avails to create the idea that we see what we do not see, but infer, though we do see something. The discovery of illusion only puts us on our guard against assuming more in our perceptions than is actually there. It forces on us the discrimination between judgments that represent a correct adjustment between external influences and internal activities and judgments that distort or add to the data of sense-perception.
What the criterion is that enables us to correct illusions need not he discussed at length. This was indicated in an earlier chapter, where it was stated to be the correction of one sense by the perception of another, or the measurement of the present impression against the totality of one's normal and repeated experience.
The most important point, however, is the distinction between organic and functional illusions. This is important because so much is made out of the phenomena of illusion generally in the problems of psychical research. In the study of residual mental phenomena the critic reminds us of our liability to illusion, and while this has not only to be admitted as well as urged as a caution, it is quite as important to know when this objection actually applies to certain allegations. We are of course exposed to illusions in psychic experiences as well as in any other phenomena, but it is important to inquire always what the types of illusion are in these experiences, and to ascertain these we must know what the phenomena are which are reputed to represent supernormal realities. But we cannot reproach them with illusion unless we distinguish the type of illusion which is chargeable in the case. Organic illusions of the type discussed will hardly enter into the problem. They represent universal and normal perception, especially those involving mathematical and diagrammatic figures. They indicate certain normal functions misadjusted to the circumstances under which they occur, and are necessary illusions, so to speak, occurring in all normal experience, and not correctible at all in sensory phenomena, but only in respect of the associations and judgments occurring at the time. They are not primarily misinterpretations of facts, but are exceptional facts or involve the operation of sensory functions other than inference and association. The phenomena with which they are connected do not pretend to be sporadic and occurring to only specially endowed persons or special conditions of all persons, but to all normal experience. No application of our liability to them can be made to such phenomena as attract the attention of the psychic researcher interested in the supernormal.
It is somewhat different with functional illusions, though some of them are complicated with the organic. Functional illusions, as we have seen, are primarily such as are influenced largely by subjective agencies and represent the misinterpretation or distortion of sensations by such facts as expectancy, suggestion, emotional states, and any mental preoccupation which involves intensity of interest in the meaning of experience. These illusions take us at least to the border-line of all those considerations which make up scientific method. Many of them, however, and especially such as are closely related to and involve organic tendencies, will have little place in the cautions necessary to observe in the usual phenomena claiming a supernormal interest. All illusions affected by indistinctness of impression and by expectancy will have a pertinence in the problems of psychic research, as understanding our liability to them will protect us against their influence on our convictions. But the real and most important errors in this field are due to other sins than illusions. These we shall discuss in their place. All that I would make clear at present is the fact that illusion as defined and discussed above has a very limited application to the problems of psychic research, though it may be related to many of the alleged phenomena claiming a "supernatural" character. I think, however, that ignorance in regard to scientific method is a more important factor in these problems than our liability to illusion.
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