PREFACE.
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 to a strictly scientific treatment, that is to say, the description 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.
_Hampstead, 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 OF ILLUSIONS.
Popular and Scientific conceptions of Mind, 9, 10; Illusion and Hallucination, 11-13; varieties of Immediate Knowledge, 13-16; four-fold 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; recognition 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.
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; Paraesesthesia, 67, 68; _rationale_ of organically conditioned Illusions, 68, 69.
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_:-- Misinterpretation 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 transformation 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 experiences, 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 Pro-adjustment or Expectation, 106-109; subordination of Sense-impression to Preperception, 109-111; transition from Illusion to Hallucination, 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.
CHAPTER VII.
DREAMS.
Mystery of sleep, 127, 128; theories of Dreams, 128, 129; scientific explanation of Dreams, 129, 130.
_Sleep and Dreaming_:--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 Dream as Hallucination_:--Results of direct central stimulation 151-153; indirect central stimulation and association, 153-155.
_The Form and Structure of Dreams_:--The incoherence of Dreams explained, 156-161; coherence and unity of Dream as effected (a) by coalescence and transformation of images, 161-163; (b) by aground-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.
CHAPTER VIII.
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: Passive 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; AEsthetic Intuition, 213; Subjective Impressions of beauty misinterpreted, 213-216; analogous Emotional Intuitions, 216, 217; Insight, its nature, 217-220; Passive Illusions of Insight, 220-222; Active Illusions of Insight: projection of individual 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 forgetfulness, 261-264; confusion of distinct recollections, 264-266; Active Illusion: influence of present imaginative activity, 266-269; exaggeration 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.
_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; fallibility and trustworthiness of Memory, 290-292.
NOTE .-- Momentary Illusions of Self-consciousness, 293.
CHAPTER XI.
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; Expectation of new kinds of experience, 301, 302; Permanent Expectations 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 representations of classes of things, 322, 323; and in our views of the world as a whole, 323, 324; tendency of belief towards divergence, 325; 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 conception 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 implications, 351, 352; Fundamental Intuitions and modern psychology, 352; psychology as positive science and as philosophy, 353-355; points of resemblance between acknowledged Illusions and Fundamental Intuitions, 355, 356; question of origin, and question of validity, 356, 357; attitude of scientific mind towards philosophic scepticism, 357-360; Persistent Intuitions must be taken as true, 360, 361.
~ILLUSIONS.~.