Illusions: a psychological study
Sully, James, 1842-1923
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JAMES SULLY, AUTIIOK OF "SENSATION ANI> INTUITION," " TESSIMISM," ETC. NEW YOKlt: D. APPLETON AND COMPANY, 1, 3, and 5 BOND STKEET. 1881. 9T "7l- ^3 PBEFACE. The present volume takes a wide survey of the field of error, embracing in its view not only the illusions of sense dealt with in treatises on physiological optics, etc., but also other errors familiarly known as illusions, and resembling the former in their structure and mode of origin. I have throughout endeavoured to keep o a strictly scientific treatment, that is to say, the lescription and classification of acknowledged errors, and the explanation of these by a reference to their psychical and physical conditions. At the same time, I was not able, at the close of my exposition, to avoid pointing out how the psychology leads on to the philosophy of the subject. Some of the chapters were first roughly sketched out in articles published in magazines and reviews ; but these have been not only greatly enlarged, but, to a considerable extent, rewritten. J.S. Eampstead, April, 1881. CONTENTS. CHAPTER I. THE STUDY OF ILLUSION. Vulgar idea of Illusion, 1, 2 ; Psychological treatment of subject, 3, 4 ; definition of Illusion, 4-7; Philosophic extension of idea, 7, 8. CHAPTER II. THE CLASSIFICATION OP ILLUSIONS. Popular and Scientific conceptions of Mind, 9, 10 ; Illusion and Halluci- nation, 11-13 ; varieties of Immediate Knowledge, 13-16 ; fourfold division of Illusions, 16-18. CHAPTER III. ILLUSIONS OF PERCEPTION : GENERAL. Psychology of Perception : — The Psychological analysis of Perception, 19, 20 ; Sensation and its discrimination, etc., 20, 21 ; interpretation of Sensation, 22, 23; construction of material object, 23, 24; recogni- tion of object, specific and individual, 24-27 ; Preperception and Perception, 27-31 ; Physiological conditions of Perception, 31-33 ; Visual and other Sense-perception, 33, 34. Illusions of Perception : — Illusion of Perception defined, 35-38; sources of .Sense-illusion, 38-40 : (a) confusion of Sense-impression, 40- 44 ; (b) misinterpretation of Sense-impression, 44; Passive and Active misinterpretation, 44-46 ; Passive Illusions as organically and extra- organically conditioned, 46-49. Vlll CONTENTS. CHAPTER IV. illusions of perception — continued. A. Passive Illusions (a) as determined by the Organism. Results of Limits of Sensibility : — Relation of quantity of Sensation to that of Stimulus, 50-52 ; coalescence of simultaneous Sensations, 52-55 ; after-effect of Stimulation, 55, 56 ; effects of prolonged Stimulation, 56-58 ; Specific Energy of Nerves, 58, 59 ; localization of Sensation, 59-62 ; Subjective Sensations, 62-64. Results of Variation of Sensibility : — Rise and fall of Sensibility, 64-67 ; Paresthesia, 67, 68 ; rationale of organically conditioned Illusions, CHAPTER V. illusions of perception — continued. A. Passive Illusions (b) as determined by the Environment. Exceptional Relation of Stimulus to Organ : — Displacement of organ, etc., 70-72. Exceptional Arrangement of Circumstances in the Environment : — Mis- interpretation of the direction and movement of objects, 72-75; misperception of Distance, 75, 76 ; Illusions of depth, "relief, and solidity in Art, 77-81; Illusions connected with the perception of objects through transparent coloured media, 82-84; visual trans- formation of concave into convex form, 84-86 ; false recognition of objects, 86, 87 ; inattention to Sense-impression in Recognition, 87-91 ; suggestion taking the direction of familiar recurring ex. periences, 91, 92. CHAPTER VI. illusions of perception— continued. B. Active Illusions. Preperception and Illusion, 93-95. Voluntary Preperception : — Choice of interpretation in the case of visible movement, 95, 96 ; and in the case of flat projections of form, 96-98 ; capricious interpretation of obscure impressions, 99, 100. Involuntary Preperception : — Effects of permanent Predisposition, 101, 102; effects of partial temporary Preadjustment, 102-105; complete Pre- CONTENTS. ix adjustment or Expectation, 106-109; subordination of Sense-impres- sion to Preperception, 109-111 ; transition from Illusion to Halluci- nation, 111, 112 ; rudimentary Hallucinations, 112-114 ; developed Hallucinations, 114-116; Hallucination in normal life, 116, 117; Hallucinations of insanity, 118-120 ; gradual development of Sense- illusions, and continuity of normal and abnormal life, 120—123 ; Sanity and Insanity distinguished, 123-126. CHAPTEE VII. Mystery of sleep, 127, 128; theories of Dreams, 128, 129; scientific explanation of Dreams, 129, 130. Sleep and Breaming : — Condition of organism during sleep, 131, 132; Are the nervous centres ever wholly inactive during sleep? 132—134 nature of cerebral activity involved in Dreams, 134-136 ; psychical conditions of Dreams, 136-138. The Dream as Illusion : — External Sense-impressions as excitants of Dream-images, 139-143 ; internal " subjective" stimuli in the sense- organs, 143-145 ; organic sensations, 145-147 ; how sensations are exaggerated in Dream-interpretation, 147-151. The Bream as Hallucination : — Eesults of direct central stimulation 151-153 ; indirect central stimulation and association, 153-155. The Form and Structure of Breams : — The incoherence of Dreams ex- plained, 156-161 ; coherence and unity of Dream as effected (a) by coalescence and transformation of images, 161-163 ; (b) by a ground- tone of feeling, 164-168 ; (c) by the play of associative dispositions, 168-172 ; (d) by the activities of selective attention stimulated by the rational impulse to connect and to arrange, 172—176 ; examples of Dreams, 176-179 ; limits of intelligence and rational activity in Dreams, 180-182 ; Dreaming and mental disease, 182, 183 ; After- dreams and Apparitions, 183-185. Note.— The Hypnotic Condition, 185-188. CHAPTEE Vm. ILLUSIONS OF INTROSPECTION. Illusions of Introspection defined, 189-192 ; question of the possibility of illusory Introspection, 192-194; incomplete grasp of internal feelings as such, 194-196 ; misobservation of internal feelings : Pas- CONTENTS. sive Illusions, 196-199 ; Active Illusions, 199-202 ; malobservation of subjective states, 202-205 ; Illusory Introspection in psychology and philosophy, 205-208; value of the Introspective method. 208-211. CHAPTER IX. OTHER QUASI-PRESENTATIVE ILLUSIONS: ERRORS OF INSIGHT. Emotion and Perception, 212 ; .^Esthetic Intuition, 213 ; Subjective Im- pressions of beauty misinterpreted, 213-216 ; analogous Emotional Intuitions, 216, 217 ; Insight, its nature, 217-220 j Passive Illusions of Insight, 220-222 ; Active Illusions of Insight : projection of indi. vidual feelings, 222-224; the poetic transformation of nature, 224- 226 ; special predispositions as falsifying Insight, 226-228 ; value of faculty of Insight, 228-230. CHAPTER X. ILLUSIONS OF MEMORY. Vulgar confidence in Memory, 231-233 ; definition of Memory, 233-235 ; Psychology of Memory, 235-237 ; Physiology of Memory, 237, 238 ; Memory as localization in the past, 238-241 ; Illusions of Memory classified, 241-245. (1) Illusions of Time-Perspective : — (a) Definite Localization of events : constant errors in retrospective estimate of time, 245-249 ; varying errors : estimate of duration during a period, 249-251; variations in retrospective estimate of duration, 251-256. (b) Indefinite Localization : effect of vividness of mnemonic image on the apparent distance of events, 256-258 ; isolated public events, 258, 259 ; active element in errors of Localization, 259-261. (2) Distortions of Memory : — Transformation of past through forgetful- ness, 261-264 ; confusion of distinct recollections, 264-266 ; Active Illnsion : influence of present imaginative activity, 266-269 ; exagge- ration in recollections of remote experiences, 269, 270 ; action of present feeling in transforming past, 270, 271. (3) Hallucinations of 'Memory : — Their nature, 271-273; past dreams taken for external experiences, 273-277 ; past waking imagination taken for external reality, 277^280 ; recollection of prenatal ancestral experience, 280, 281 ; filling up gaps in recollection, 281-283. CONTENTS. XI Illusions connected with Personal Identity : — Illusions of Memory and Sense of identity, 283, 284 ; idea of permanent self, how built up, 285-287; disturbances of sense of identity, 287-290 j fallibility and trustworthiness of Memory, 290-292. Note. — Momentary Illusions of Self-consciousness, 293. CHAPTEE XL ILLUSIONS OF BELIEF. Belief as Immediate or Intuitive, 294-296 ; simple and compound Belief, 296. A. Simple Illusory Belief : — (1) Expectation : its nature, 297, 298 ; Is Expectation ever intuitive ? 298 ; Expectation and Inference from the past, 299-301 ; Expec- tation of new kinds of experience, 301, 302 ; Permanent Expecta- tions of remote events, 302 ; misrepresentation of future duration, 302—305 ; Imaginative transformation of future, 305-307. (2) Quasi-Expectations : anticipation of extra-personal experiences, 307, 308 ; Retrospective Beliefs, 308-312. B. Compound Illusory Belief: — (1) Representations of permanent things : their structure, 312 ; our representations of others as illusory, 312-315 ; our representation of ourselves as illusory, 315; Illusion of self-esteem, 316-318; genesis of illusory opinion of self, 318-322 ; Illusion in our repre- sentations of classes of things, 322, 323 ; and in our views of the world as a whole, 323, 324 ; tendency of belief towards divergence, 325 j and towards convergence, 326, 327. CHAPTER XII. RESULTS. Range of Illusion, 328-330 ; nature and causes of Illusion in general, 331-334 ; Illusion identical with Fallacy, 334 ; Illusion as abnormal, 336, 337; question of common error, 337-339; evolutionist's con- ception of error as maladaptation, 339-344; common intuitions tested only by philosophy, 344 ; assumptions of science respecting external reality, etc., 344-346 ; philosophic investigation of these assumptions, 346-348 ; connection between scientific and philosophic consideration of Illusion, 348-350; correction of Illusion and its CONTENTS. implications, 351, 352 ; Fundamental Intuitions and modern psycho- logy, 352 ; psychology as positive science and as philosophy, 353-355 ; points of resemblance between acknowledged Illusions and Funda- mental Intuitions, 355, 356; question of origin-, and question of validity, 356, 357 ; attitude of scientific mind towards philosophic scepticism, 357-360 j Persistent Intuitions must be taken as true, 360, 361. ILLUSIONS. CHAPTEE I. THE STUDY OF ILLUSION. Common sense, knowing nothing of fine distinctions, is wont to draw a sharp line between the region of illusion and that of sane intelligence. To be the victim of an illusion is, in the popular judgment, to be excluded from the category of rational men. The term at once calls up images of stunted figures with ill-developed brains, half-witted creatures, hardly dis- tinguishable from the admittedly insane. And this way of thinking of illusion and its subjects is strength- ened by one of the characteristic sentiments of our age. The nineteenth century intelligence plumes itself on having got at the bottom of mediaeval visions and church miracles, and it is wont to commiserate the feeble minds that are still subject to these self- deceptions. According to this view, illusion is something essen- tially abnormal and allied to insanity. And it would seem to follow that its nature and origin can be best 2 THE STUDY OF ILLUSION. studied by those whose speciality it is to observe the phenomena of abnormal life. Scientific procedure has in the main conformed to this distinction of common sense. The phenomena of illusion have ordinarily been investigated by alienists, that is to say, physicians who are brought face to face with their most striking forms in the mentally deranged. While there are very good reasons for this treat- ment of illusion as a branch of mental pathology, it is by no means certain that it can be a complete and exhaustive one. Notwithstanding the flattering supposition of common sense, that illusion is essentially an incident in abnormal life, the careful observer knows well enough that the case is far otherwise. There is, indeed, a view of our race diametrically opposed to the flattering opinion referred to above, namely, the humiliating judgment that all men habitually err, or that illusion is to be regarded as the natural condition of mortals. This idea has found expression, not only in the cynical exclamation of the misanthropist that most men are fools, but also in the cry of despair that sometimes breaks from the weary searcher after absolute truth, and from the poet when impressed with the unreality of his early ideals. Without adopting this very disparaging opinion of the intellectual condition of mankind, we must recognize the fact that most men are sometimes liable to illusion. Hardly anybody is always con- sistently sober and rational in his perceptions and beliefs. A momentary fatigue of the nerves, a little mental excitement, a relaxation of the effort of atten- tion by which we continually take our bearings with POPULAR IDEA OF ILLUSION. 3 respect to the real world about us, will produce just the same kind of confusion of reality and phantasm which we observe in the insane. To give but an example : the play of fancy which leads to a detection of animal and other forms in clouds, is known to be an occupation of the insane, and is rightly made use of by Shakespeare as a mark of incipient mental aberration in Hamlet ; and yet this very same occupation is quite natural to children, and to imaginative adults when they choose to throw the reins on the neck of their phantasy. Our luminous circle of rational perception is surrounded by a misty penumbra of illusion. Common sense itself may be said to admit this, since the greatest stickler for the enlightenment of our age will be found in practice to accuse most of his acquaintance at some time or another of falling into illusion. If illusion thus has its roots in ordinary mental life, the study of it would seem to belong to the physiology as much as to the pathology of mind. We may even go further, and say that in the analysis and explana- tion of illusion the psychologist may be expected to do more than the physician. If, on the one hand, the latter has the great privilege of observing the pheno- mena in their highest intensity, on the other hand, the former has the advantage of being familiar with the normal intellectual process which all illusion simulates or caricatures. To this it must be added that the physician is naturally disposed to look at illusion mainly, if not exclusively, on its practical side, that is, as a concomitant and symptom of cerebral disease, which it is needful to be able to recognize. The psychologist has a different interest in the subject, 4 THE STUDY OF ILLUSION. being specially concerned to understand the mental antecedents of illusion and its relation to accurate perception and belief. It is pretty evident, indeed, that the' phenomena of illusion form a region common to the psychologist and the mental pathologist, and that the complete elucidation of the subject will need the co-operation of the two classes of investigator. In the present volume an attempt will be made to work out the psychological side of the subject ; that is. to say, illusions will be viewed in their relation to the process of just and accurate perception. In the carrying out of this plan our principal attention will be given to the manifestations of the illusory impulse in normal life. At the same time, though no special acquaintance with the pathology of the subject will be laid claim to, frequent references will be made to the illusions of the insane. Indeed, it will be found that the two groups of phenomena — the illusions of the normal and of the abnormal condition — are so similar, and pass into one another by such insensible grada- tions, that it is impossible to discuss the one apart from the other. The view of illusion which will be adopted in this work is that it constitutes a kind of borderland between perfectly sane and vigorous mental life and dementia. And here at once there forces itself on our atten- tion the question, What exactly is to be understood by the term " illusion " ? In scientific works treating of the pathology of the subject, the word is confined to what are specially known as illusions of the senses, that is to say, to false or illusory perceptions. And there is very good reason for this limitation, since such WHAT IS ILLUSION ? 5 illusions of the senses are the most palpable and striking symptoms of mental disease. In addition to this, it must be allowed that, to the ordinary reader, the term first of all calls up this same idea of a decep- tion of the senses. At the same time, popular usage has long since extended the term so as to include under it errors which do not counterfeit actual perceptions. We commonly speak of a man being under an illusion respecting himself when he has a ridiculously exag- gerated view of his own importance, and in a similar way of a person being in a state of illusion with respect to the past when, through frailty of memory, he pictures it quite otherwise than it is certainly known to have been. It will be found, I think, that there is a very good- reason for this popular extension of the term. The errors just alluded to have this in common with illusions of sense, that they simulate the form of immediate or self-evident cognition. An idea held respecting ourselves or respecting our past history does not depend on any other piece of knowledge ; in other words, is not adopted as the result of a process of reasoning. What I believe with reference to my past history, so far as I can myself recall it, I believe instantaneously and immediately, without the inter- vention of any premise or reason. Similarly, our notions of ourselves are, for the most part, obtained apart from any process of inference. The view which a man takes of his own character or claims on society he is popularly supposed to receive intuitively by a mere act of internal observation. Such beliefs may 6 THE STUDY OF ILLUSION. not, indeed, have all the overpowering force which belongs to illusory perceptions, for the intuition of something by the senses is commonly looked on as the most immediate and irresistible kind of knowledge. Still, they must be said to come very near illusions of sense in the degree of their self-evident certainty. Taking this view of illusion, we may provisionally define it as any species of error which counterfeits the form of immediate, self-evident, or intuitive knowledge, whether as sense-perception or otherwise. Whenever a thing is believed on its own evidence and not as a conclusion from something else, and the thing then believed is demonstrably wrong, there is an illusion. The term would thus appear to cover all varieties of error which are not recognized as fallacies or false inferences. If for the present we roughly divide all our knowledge into the two regions of primary or intuitive, and secondary or inferential knowledge, we see that illusion is false or spurious knowledge of the first kind, fallacy false or spurious knowledge of the second kind. At the same time, it is to be remembered that this division is only a very rough one. As will appear in the course of our investigation, the same error may be called either a fallacy or an illusion, according as we are thinking of its original mode of production or of the form which it finally assumes; and a thorough-going psychological analysis of error may discover that these two classes are at bottom very similar. As we proceed, we shall, I think, find an ample justification for our definition. We shall see that such illusions as those respecting ourselves or the DEFINITION OF ILLUSION. 7 past arise by very much the same mental processes as those which are discoverable in the production of illusory perceptions ; and thus a complete psychology of the one class will, at the same time, contain the explanation of the other classes. The reader is doubtless aware that philosophers have still further extended the idea of illusion by seeking to bring under it beliefs which the common sense of mankind has always adopted and never begun to suspect. Thus, according to the idealist, the popu- lar notion (the existence of which Berkeley, however, denied) of an external world, existing in itself and in no wise dependent on our perceptions of it, resolves itself into a grand illusion of sense. At the close of our study of illusions we shall return to this point. We shall there inquire into the connection between those illusions which are popularly recognized as such, and those which first come into view or appear to do so (for we must not yet assume that there are such) after a certain kind of philosophic reflection. And some attempt will be made to de- termine roughly how far the process of dissolving these substantial beliefs of mankind into airy phantasms may venture to go. Foi the present, however, these so-called illusions in philosophy will be ignored. It is plain that illusion exists only in antithesis to real knowledge. This last must be assumed as something above all question. And a rough and provisional, though for our purpose sufficiently accurate, demarcation of the regions of the real and the illusory seems to coincide with the line which common sense draws between what all normal 8 THE STUDY OF ILLUSION. men agree in holding and what the individual holds, whether temporarily or permanently, in contradiction to this. For our present purpose the real is that which is true for all. Thus, though physical science may tell us that there is nothing corresponding to our sensations of colour in the world of matter and motion which it conceives as surrounding us; yet, inasmuch as to all men endowed with the normal colour-sense the same material objects appear to have the same colour, we may speak of any such perception as practically true, marking it off from those plainly illusory perceptions which are due to some subjective cause, as, for example, fatigue of the retina. To sum up: in treating of illusions we shall assume, what science as distinguished from philosophy is bound to assume, namely, that human experience is consistent ; that men's perceptions and beliefs fall into a consensus. From this point of view illusion is seen to arise through some exceptional feature in the situa- tion or condition of the individual, which, for the time, breaks the chain of intellectual solidarity which under ordinary circumstances binds the single member to the collective body. Whether the common ex- perience which men thus obtain is rightly interpreted is a question which does not concern us here. For our present .purpose, which is the determination and explanation of illusion as popularly understood, it is sufficient that there is this general consensus of belief, and this may provisionally be regarded as at least practically true. CHAPTEE II. THE CLASSIFICATION OF ILLUSIONS. If illusion is the simulation of immediate knowledge, the most obvious mode of classifying illusions would appear to be according to the variety of the knowledge which they simulate. Now, the popular psychology that floats about in the ordinary forms of language has long since dis- tinguished certain kinds of unreasoned or uninferred knowledge. Of these the two best known are per- ception and memory. When I see an object before me, or when I recall an event in my past experience, I am supposed to grasp a piece of knowledge directly, to know something immediately, and not through the medium of something else. Yet I know differently in the two cases. In the first I know by what is called a presentative process, namely, that of sense-perception ; in the second I know by a representative process, namely, that of reproduction, or on the evidence of memory. In the one case the object of cognition is present to my perceptive faculties ; in the other it is recalled by the power of memory. Scientific psychology tends, no doubt, to break down some of these popular distinctions. Just as the zoologist 10 THE CLASSIFICATION OF ILLUSIONS. sometimes groups together varieties of animals which the unscientific eye would never think of connecting, so the psychologist may analyze mental operations which appear widely dissimilar to the popular mind,' and reduce them to one fundamental process. Thus recent psychology draws no sharp distinction between per- ception and recollection. It finds in both very much the same elements, though combined in a different way Strictly speaking, indeed, perception must be defined as a presentative-representative operation. To the psychologist it comes to very much the same thing whether, for example, on a visit to Switzerland, our minds are occupied in perceiving the distance of a mountain or in remembering some pleasant excursion which we made to it on a former visit. In both cases there is a reinstatement of the past, a reproduction of earlier experience, a process of adding to a present impression a product of imagination — taking this word in its widest sense. In both cases the same laws of reproduction or association are illustrated. Just as a deep and exhaustive analysis of the intellectual operations thus tends to identify their various forms as they are distinguished by the popular mind, so a thorough investigation of the flaws in these operations, that is to say, the counterfeits of knowledge, will probably lead to an identification of the essential mental process which underlies them. It is apparent, for example, that, whether a man projects some figment of his imagination into the external world, giving it, present material reality, or whether (if I may be allowed the term) he retrojects ,it into the dim region of the past, and takes it for a reality that has been POPULAR AND SCIENTIFIC CLASSIFICATION. 11 he is committing substantially the same blunder. The source of the illusion in both cases is one and the same. It might seem to follow from this that a scientific discussion of the subject would overlook the obvious distinction between illusions of perception and those of memory ; that it would attend simply to differences in the mode of origination of the illusion, whatever its external form. Our next step, then, would appear to be to determine these differences in the mode of pro- duction. That there are differences in the origin and source of illusion is a fact which has been fully recognized by those writers who have made a special study of sense- illusions. By these the term illusion is commonly employed in a narrow, technical sense, and opposed to hallucination. An illusion, it is said, must always have its starting-point in some actual impression, whereas a hallucination has no such basis. Thus it is an illusion when a man, under the action of terror, takes a stump of a tree, whitened by the moon's rays, for a ghost. It is a hallucination when an imaginative person so vividly pictures to himself the form of some absent friend that, for the moment, he fancies himself actually beholding him. Illusion is thus a partial displacement of external fact by a fiction of the imagination, while hallucination is a total displacement. This distinction, which has been adopted by the majority of recent alienists,1 is a valuable one, and 1 A history of the distinction is given by Brierre de Boiamont, in his work On Illusions (translated by E. T. Huline, 1859). He says that Arnold (1806) first defined hallucination, and distinguished it 12 THE CLASSIFICATION OF ILLUSIONS. must not be lost sight of here. It would seem, from a psychological point of view, to be an important cir- cumstance in the genesis of a false perception whether the intellectual process sets out from within or from without. And it will be found, moreover, that this distinction may be applied to all the varieties of error which I propose to consider. Thus, for example, it will be seen further on that a false recollection may set out either from the idea of some actual past occurrence or from a present product of the imagination. It is to be observed, however, that the line of separation between illusion and hallucination, as thus defined, is a very narrow one. In by far the largest number of hallucinations it is impossible to prove that there is no modicum of external agency co-operating in the production of the effect. It is presumable, indeed, that many, if not all, hallucinations have such a basis of fact. Thus, the madman who projects his internal thoughts outwards in the shape of external voices may, for aught we know, be prompted to do so in part by faint impressions coming from the ear, the result of those slight stimulations to which the organ is always exposed, even in profound silence, and which in his case assume an exaggerated intensity. And ever if it is clearly made out that there are hallucinations in the strict sense, that is to say, false perceptions which are wholly due to internal causes, it must be conceded that illusion shades off into hallucination by steps which it is impossible for science to mark. In from illusion. Esquirer, in his work, Des Maladies Mentales (183S), may bo said to have fixed the distinction. (See Hunt's translation, 1845, p. 111.) ILLUSION AND HALLUCINATION. 13 many cases it must be left an open question whether the error is to be classed as an illusion or as a hallu- cination.1 For these reasons, I think it best not to make the distinction between illusion and hallucination the leading principle of my classification. However im- portant psychologically, it does not lend itself to this purpose. The distinction must be kept in view and illustrated as far as possible. Accordingly, while in general following popular usage and employing the term illusion as the generic name, I shall, when con- venient, recognize the narrow and technical sense of the term as answering to a species co-ordinate with hallucination. Departing, then, from what might seem the ideally best order of exposition, I propose, after all, to set out with the simple popular scheme of faculties already referred to. Even if they are, psychologically con- sidered, identical operations, perception and memory are in general sufficiently marked off by a speciality in the form of the operation. Thus, while memory is the reproduction of something with a special reference of consciousness to its past existence, perception is the reproduction of something with a special reference to its present existence as a part of the presented object. In other words, though largely representative when viewed as to its origin, perception is presentative in relation to the object which is supposed to be im- 1 This fact has been fully recognized by writers on the pathology cf the subject; for example, Griesinger, Mental Pathology and Thera- peutics (London, 1867), p. 84; Baillarger, article, "Des Hallucina- tions," in the Me'moires de I'Academie lloyale de Me'decine, torn. xii. p. 273, etc. ; Wundt, Fhysiologischc Psychologie, p. 653. 2 14 THE CLASSIFICATION OF ILLUSIONS. mediately present to the mind at the moment.1 Hence the convenience of recognizing the popular classification, and of making it our starting-point in the present case. All knowledge which has any appearance of being directly reached, immediate, or self-evident, that is to say, of not being inferred from other knowledge, may be divided into four principal varieties : Internal Perception or Introspection of the mind's own feelings ; External Perception ; Memory ; and Belief, in so far as it simulates the form of direct knowledge. The first is illustrated in a man's consciousness of a present feeling of pain or pleasure. The second and the third kinds have already been spoken of, and are too familiar to require illustration. It is only needful to remark here that, under perception, or rather in close con- junction with it, I purpose dealing with the knowledge of other's feelings, in so far as this assumes the aspect of immediate knowledge. The term belief is here used to include expectations and any other kinds of conviction that do not fall under one of the other heads. An instance of a seemingly immediate belief would be a prophetic prevision of a coming disaster, or a man's unreasoned persuasion as to his own powers of performing a difficult task. It is, indeed, said by many thinkers that there are no legitimate immediate beliefs ; that all our expecta- tions and other convictions about things, in so far as they are sound, must repose on other genuinely immediate 1 I here touch on the distinction between tho psychological and the philosophical view of perception, to bo brought out more fully by-and-by. VARIETIES OF IMMEDIATE KNOWLEDGE, 15 knowledge, more particularly sense-perception and memory. This difficult question need not be discussed here. It is allowed by all that there is a multitude of beliefs which we hold tenaciously and on which we are ready to act, which, to the mature mind, wear the appearance of intuitive truths, owing their cogency to nothing beyond themselves. A man's belief in his own merits, however it may have been first obtained, is as immediately assured to him as his recognition of a real object in the act of sense-perception. It may be added that many of our every-day working beliefs about the world in which we live, though presumably derived from memory and perception, tend to lose all traces of their origin, and to simulate the aspect of intuitions. Thus the proposition that logicians are in the habit of pressing on our attention, that " Men are mortal," seems, on the face of it, to common sense to be something very like a self-evident truth, not depend- ing on any particular facts of experience. In calling these four forms of cognition immediate, I must not, however, be supposed to be placing them on the same logical level. It is plain, indeed, to a reflec- tive mind that, though each may be called immediate in this superficial sense, there are perceptible differences in the degree of their immediacy. Thus it is manifest, after a moment's reflection, that expectation, so far as it is just, is not primarily immediate in the sense in which purely presentative knowledge is so, since it can be shown to follow from something else. So a general proposition, though through familiarity and innumerable illustrations it has acquired a self-evident character, is seen with a very little inspection to be 16 THE CLASSIFICATION OF ILLUSIONS. less fundamentally and essentially so than the proposi- tion, " I am now feeling pain ; " and it will be found that even with respect to memory, when the remem- bered event is at all remote, the process of cognition approximates to a mediate operation, namely, one of inference. What the relative values of these different kinds of immediate knowledge are is a point which will have to be touched on at the end of our study. Here it must suffice to warn the reader against the supposition that this value is assumed to be identical. It might seem at a first glance to follow from this four-fold scheme of immediate or quasi-immediate knowledge that there are four varieties of illusion. And this is true in the sense that these four heads cover all the main varieties of illusion. If there are only four varieties of knowledge which can lay any claim to be considered immediate, it must be that every illusion will simulate the form of one of these varieties, and so be referable to the corresponding division. But though there are conceivably these four species of illusion, it does not follow that there are any actual instances of each class forthcoming. This we cannot determine till we have investigated the nature and origin of illusory error. For example, it might be found that introspection, or the immediate inspection of our own feelings or mental states, does not supply the conditions necessary to the production of such error. And, indeed, it is probable that most persons, antecedently to inquiry, would be disposed to say that to fall into error in the observation of what is actually going on in our own minds is impossible. VARIETIES OF ILLUSION. 17 With the exception of this first division, however, this scheme may easily be seen to answer to actual phenomena. That there are illusions of perception is obvious, since it is to the errors of sense that the term illusion has most frequently been confined. It is hardly less evident that there, are illusions of memory. The peculiar difficulty of distinguishing between a past real event and a mere phantom of the imagina- tion, illustrated in the exclamation, " I either saw it or dreamt it," sufficiently shows that memory is liable to be imposed on. Finally, it is agreed by all that the beliefs we are wont to regard as self-evident are sometimes erroneous. When, for example, an imagina- tive woman says she knows, by mere intuition," that something interesting is going to happen, say the arrival of a favourite friend, she is plainly running the risk of being self-deluded. So, too, a man's esti- mate of himself, however valid for him, may turn out to be flagrantly false. In the following discussion of the subject I shall depart from the above order in so far as to set out with illusions of sense-perception. These are well ascer- tained, forming, indeed, the best-marked variety. And the explanation of these has been carried much further than that of the others. Hence, according to the rule to proceed from the known to the unknown, there will be an obvious convenience in examining these first of all. After having done this, we shall be in a position to inquire whether there is anything analogous in the region of introspection or internal perception. Our study of the errors of sense-per- ception will, moreover, prove the best preparation for 18 THE CLASSIFICATION OF ILLUSIONS. an inquiry into the. nature and mode of production of the remaining two varieties.1 I would add that, in close connection with the first division, illusions of perception, I shall treat the subtle and complicated phenomena of dreams. Al- though containing elements which ought, according to strictness, to be brought under one of the other heads, they are, as their common appellation, " visions," shows, largely simulations of external, and more especially visual, perception. Dreams are no doubt sharply marked off from illusions of sense-perception by a number of special circumstances. Indeed, it may be thought that they cannot be adequately treated in a work that aims primarily at investigating the illusions of normal life, and should rather be left to those who make the pathological side of the subject their special study. Yet it may, perhaps, be said that in a wide sense dreams are a feature of normal life. And, however this be, they have quite enough in common with other illusions of perception to justify us in dealing with them in close connection with these. 1 It might even be urged that the order here adopted is scientifically the best, since sense-perception is the earliest form of knowledge, introspected facts being known only in relation to perceived facts. But if the mind's knowledge of its own states is thus later in time, it is earlier in the logical order, that is to say, it is the most strictly pre- sentative form of knowledge. CHAPTEE III. ILLUSIONS OF PERCEPTION : GENERAL. 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 know- ledge 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 per- ception. I may observe that in this analysis of perception I shall endeavour to keep to known facts, namely, the 20 ILLUSIONS OF PE11CEPTION. 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 under- stand 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 con- crete 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 opera- tion, 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 x 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 1 Here and elsewhere I use the word " impression" for the whole complex of sensation which is present at the moment. It may, perhaps, not be unnecessary to add that, in employing this term, I am. making no assumption about the independent existence of external objects. 