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ILLUSIONS
_A PSYCHOLOGICAL STUDY_
BY
JAMES SULLY
AUTHOR OF "SENSATION AND INTUITION," "PESSIMISM," ETC.
THIRD EDITION
LONDON
KEGAN PAUL, TRENCH & CO., 1, PATERNOSTER SQUARE
1887
(_The rights of translation and of reproduction are reserved_)
~THE INTERNATIONAL SCIENTIFIC SERIES.~
VOL. XXXIV.
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.~
CHAPTER 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 distinguishable from the admittedly insane. And this
way of thinking of illusion and its subjects is strengthened 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 essentially abnormal and
allied to insanity. And it would seem to follow that its nature and
origin can be best 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 treatment 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 consistently
sober and rational in his perceptions and beliefs. A momentary fatigue
of the nerves, a little mental excitement, a relaxation of the effort of
attention by which we continually take our bearings with 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
explanation 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 phenomena 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, 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
gradations, 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 border-land between perfectly sane and vigorous
mental life and dementia.
And here at once there forces itself on our attention 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 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 deception 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 exaggerated 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 intervention 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 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 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 popular 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 determine roughly how far the process of dissolving these
substantial beliefs of mankind into airy phantasms may venture to go.
For 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 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 situation 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 experience 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.
CHAPTER 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 distinguished certain kinds of unreasoned or
uninferred knowledge. Of these the two best known are perception 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 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 perception 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 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 production.
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 must not be lost sight of here. It
would seem, from a psychological point of view, to be an important
circumstance 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 even 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 many cases it must be left an open
question whether the error is to be classed as an illusion or as a
hallucination.[2]
For these reasons, I think it best not to make the distinction between
illusion and hallucination the leading principle of my classification.
However important 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
convenient, 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 considered, 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
immediately present to the mind at the moment.[3] 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 conjunction 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 expectations and other convictions about
things, in so far as they are sound, must repose on other genuinely
immediate 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 depending 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 reflective 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 less fundamentally and essentially so than the
proposition, "I am now feeling pain;" and it will be found that even
with respect to memory, when the remembered 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.
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
imagination, 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 on by all that the beliefs we are wont to regard as
self-evident are sometimes erroneous. When, for example, an imaginative
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 estimate 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 ascertained, 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-perception will,
moreover, prove the best preparation for an inquiry into the nature and
mode of production of the remaining two varieties.[4]
I would add that, in close connection with the first division, illusions
of perception, I shall treat the subtle and complicated phenomena of
dreams. Although 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.
CHAPTER 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 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.
CHAPTER IV.
ILLUSIONS OF PERCEPTION--_continued_.
A. _Passive Illusions (a) as determined by the Organism._
In dealing with the illusions which are related to certain peculiarities
in the nervous organism and the laws of sensibility, I shall commence
with those which are connected with certain limits of sensibility.
_Limits of Sensibility._
To begin with, it is known that the sensation does not always answer to
the external stimulus in its degree or intensity. Thus, a certain amount
of stimulation is necessary before any sensation arises. And this will,
of course, be greater when there is little or no attention directed to
the impression, that is to say, no co-operating central reaction. Thus
it happens that slight stimuli go overlooked, and here illusion may have
its starting-point. The most familiar example of such slight errors is
that of movement. When we are looking at objects, our ocular muscles are
apt to execute very slight movements which escape our notice. Hence we
tend, under certain circumstances, to carry over the retinal result of
the movement, that is to say, the impression produced by a shifting of
the parts of the retinal image to new nervous elements, to the object
itself, and so to transform a "subjective" into an "objective" movement.
In a very interesting work on apparent or illusory movements, Professor
Hoppe has fully investigated the facts of such slight movements, and
endeavoured to specify their causes.[17]
Again, even when the stimulus is sufficient to produce a conscious
impression, the degree of the feeling may not represent the degree of
the stimulus. To take a very inconspicuous case, it is found by Fechner
that a given increase of force in the stimulus produces a less amount of
difference in the resulting sensations when the original stimulus is a
powerful one than when it is a feeble one. It follows from this, that
differences in the degree of our sensations do not exactly correspond to
objective differences. For example, we tend to magnify the differences
of light among objects, all of which are feebly illuminated, that is to
say, to see them much more removed from one another in point of
brightness than when they are more strongly illuminated. Helmholtz
relates that, owing to this tendency, he has occasionally caught
himself, on a dark night, entertaining the illusion that the
comparatively bright objects visible in twilight were self-luminous.[18]
Again, there are limits to the conscious separation of sensations which
are received together, and this fact gives rise to illusion. In general,
the number of distinguishable sensations answers to the number of
external causes; but this is not always the case, and here we naturally
fall into the error of mistaking the number of the stimuli. Reference
has already been made to this fact in connection with the question
whether consciousness can be mistaken as to the character of a present
feeling.
The case of confusing two impressions when the sensory fibres involved
are very near one another, has already been alluded to. Both in touch
and in sight we always take two or more points for one when they are
only separated by an interval that falls below the limits of local
discrimination. It seems to follow from this that our perception of the
world as a continuum, made up of points perfectly continuous one with
another may, for what we know, be illusory. Supposing the universe to
consist of atoms separated by very fine intervals, then it is
demonstrable that it would appear to our sensibility as a continuum,
just as it does now.[19]
Two or more simultaneous sensations are indistinguishable from one
another, not only when they have nearly the same local origin, but under
other circumstances. The blending of partial sensations of tone in a
_klang_-sensation, and the coalescence in certain cases of the
impressions received by way of the two retinas, are examples of this. It
is not quite certain what determines this fusion of two simultaneous
feelings. It may be said generally that it is favoured by similarity
between the sensations;[20] by a comparative feebleness of one of the
feelings; by the fact of habitual concomitance, the two sensations
occurring rarely, if ever, in isolation; and by the presence of a mental
disposition to view them as answering to one external object. These
considerations help us to explain the coalescence of the retinal
impressions and its limits, the fusion of partial tones, and so on.[21]
It is plain that this fusion of sensations, whatever its exact
conditions may be, gives rise to error or wrong interpretation of the
sense-impression. Thus, to take the points of two legs of a pair of
compasses for one point is clearly an illusion of perception. Here is
another and less familiar example. Very cold and smooth surfaces, as
those of metal, often appear to be wet. I never feel sure, after wiping
the blades of my skates, that they are perfectly dry, since they always
seem more or less damp to my hand. What is the reason of this? Helmholtz
explains the phenomenon by saying that the feeling we call by the name
of wetness is a compound sensation consisting of one of temperature and
one of touch proper. These sensations occurring together so frequently,
blend into one, and so we infer, according to the general instinctive
tendency already noticed, that there is one specific quality answering
to the feeling. And since the feeling is nearly always produced by
surfaces moistened by cold liquid, we refer it to this circumstance, and
speak of it as a feeling of wetness. Hence, when the particular
conjunction of sensations arises apart from this external circumstance,
we erroneously infer its presence.[22]
The most interesting case of illusion connected with the fusion of
simultaneous sensations, is that of single vision, or the deeply
organized habit of combining the sensations of what are called the
corresponding points of the two retinas. This coalescence of two
sensations is so far erroneous since it makes us overlook the existence
of two distinct external agencies acting on different parts of the
sensitive surface of the body. And this is the more striking in the case
of looking at solid objects, since here it is demonstrable that the
forces acting on the two retinas are not perfectly similar.
Nevertheless, such a coalescence plainly answers to the fact that these
external agencies usually arise in one and the same object, and this
unity of the object is, of course, the all-important thing to be sure
of.
This habit may, however, beget palpable illusion in another way. In
certain exceptional cases the coalescence does not take place, as when I
look at a distant object and hold a pencil just before my eyes.[23] And
in this case the organized tendency to take one visual impression for
one object asserts its force, and I tend to fall into the illusion of
seeing two separate pencils. If I do not wholly lapse into the error, it
is because my experience has made me vaguely aware that double images
under these circumstances answer to one object, and that if there were
really two pencils present I should have four visual impressions.
Once more, it is a law of sensory stimulation that an impression
persists for an appreciable time after the cessation of the action of
the stimulus. This "after sensation" will clearly lead to illusion, in
so far as we tend to think of the stimulus as still at work. It forms,
indeed, as will be seen by-and-by, the simplest and lowest stage of
hallucination. Sometimes this becomes the first stage of a palpable
error. After listening to a child crying for some time the ear easily
deceives itself into supposing that the noise is continued when it has
actually ceased. Again, after taking a bandage from a finger, the
tingling and other sensations due to the pressure sometimes persist for
a good time, in which case they easily give rise to an illusion that the
finger is still bound.
It follows from this fact of the reverberation of the nervous structures
after the removal of a stimulus, that whenever two discontinuous
stimulations follow one another rapidly enough, they will appear
continuous. This fact is a fruitful source of optical illusion. The
appearance of a blending of the stripes of colours on a rotating disc or
top, of the formation of a ring of light by swinging round a piece of
burning wood, and the illusion of the toy known as the thaumatrope, or
wheel of life, all depend on this persistence of retinal impression.
Many of the startling effects of sleight of hand are undoubtedly due in
part to this principle. If two successive actions or sets of
circumstances to which the attention of the spectator is specially
directed follow one another by a very narrow interval of time, they
easily appear continuous, so that there seems absolutely no time for the
introduction of an intermediate step.[24]
There is another limit to sensibility which is in a manner the opposite
to the one just named. It is a law of nervous stimulation that a
continued activity of any structure results in less and less psychic
result, and that when a stimulus is always at work it ceases in time to
have any appreciable effect. The common illustration of this law is
drawn from the region of sound. A constant noise, as of a mill, ceases
to produce any conscious sensation. This fact, it is plain, may easily
become the commencement of an illusion. Not only may we mistake a
measure of noise for perfect silence,[25] we may misconceive the real
nature of external circumstances by overlooking some continuous
impression.
Curious illustrations of this effect are found in optical illusions,
namely, the errors we make respecting the movement of stationary objects
after continued movement of the eyes. When, for example, in a railway
carriage we have for some time been following the (apparent) movement of
objects, as trees, etc., and turn our eyes to an apparently stationary
object, as the carpet of the compartment, this seems to move in the
contrary direction to that of the trees. Helmholtz's explanation of this
illusion is that when we suppose that we are fixing our eye on the
carpet we are really continuing to move it over the surface by reason of
the organic tendency, already spoken of, to go on doing anything that
has been done. But since we are unaware of this prolonged series of
ocular movements, the muscular feelings having become faint, we take the
impression produced by the sliding of the picture over the retina to be
the result of a movement of the object.[26]
Another limit to our sensibility, which needs to be just touched on
here, is known by the name of the specific energy of the nerves. One and
the same nerve-fibre always reacts in a precisely similar way, whatever
the nature of the stimulus. Thus, when the optic nerve is stimulated in
any manner, whether by light, mechanical pressure, or an electric
current, the same effect, a sensation of light, follows.[27] In a usual
way, a given class of nerve-fibre is only stimulated by one kind of
stimulus. Thus, the retina, in ordinary circumstances, is stimulated by
light. Owing to this fact, there has arisen a deeply organized habit of
translating the impression in one particular way. Thus, I instinctively
regard a sensation received by means of the optic nerve as one caused by
light.
Accordingly, whenever circumstances arise in which a like sensation is
produced by another kind of stimulus, we fall into illusion. The
phosphenes, or circles of light which are seen when the hinder part of
the eyeball is pressed, may be said to be illusory in so far as we
speak of them as perceptions of light, thus referring them to the
external physical agency which usually causes them. The same remark
applies to those "subjective sensations," as they are called, which are
known to have as their physical cause subjective stimuli, consisting, in
the case of sight, in varying conditions of the peripheral organ, as
increased blood-pressure. Strictly speaking, such simple feelings as
these appear to be, involve an ingredient of false perception: in saying
that we _perceive_ light at all, we go beyond the pure sensation,
interpreting this wrongly.
Very closely connected with this limitation of our sensibility is
another which refers to the consciousness of the local seat, or origin
of the impression. This has so far its basis in the sensation itself as
it is well known that (within the limits of local discrimination,
referred to above) sensations have a particular "local" colour, which
varies in the case of each of the nervous fibres by the stimulation of
which they arise.[28] But though this much is known through a difference
in the sensibility, nothing more is known. Nothing can certainly be
ascertained by a mere inspection of the sensation as to the distance the
nervous process has travelled, whether from the peripheral termination
of the fibre or from some intermediate point.
In a general way, we refer our sensations to the peripheral endings of
the nerves concerned, according to what physiologists have called "the
law of eccentricity." Thus I am said to feel the pain caused by a
bruise in the foot in the member itself. This applies also to some of
the sensations of the special senses. Thus, impressions of taste are
clearly localized in the corresponding peripheral terminations.
With respect to the sense of smell, and still more to those of hearing
and sight, where the impression is usually caused by an object at a
distance from the peripheral organ, our attention to this external cause
leads us to overlook in part the "bodily seat" of the sensation. Yet
even here we are dimly aware that the sensation is received by way of a
particular part of the sensitive surface, that is to say, by a
particular sense-organ. Thus, though referring an odour to a distant
flower, we perceive that the sensation of odour has its bodily origin in
the nose. And even in the case of hearing and sight, we vaguely refer
the impressions, as such, to the appropriate sense-organ. There is,
indeed, in these cases a double local reference, a faint one to the
peripheral organ which is acted on, and a more distinct one to the
object or the force in the environment which acts on this.
Now, it may be said that the act of localization is in itself distinctly
illusory, since it is known that the sensation first arises in
connection with the excitation of the sensory _centre_, and not of the
peripheral fibre.[29] Yet it must at least be allowed that this
localization of sensation answers to the important fact that, under
usual circumstances, the agency producing the sensation is applied at
this particular point of the organism, the knowledge of which point is
supposed by modern psychologists to have been very slowly learnt by the
individual and the race, through countless experiments with the moving
organ of touch, assisted by the eye.
Similarly, the reference of the impression, in the case of hearing and
sight, to an object in the environment, though, as we have seen, from
one point of view illusory, clearly answers to a fact of our habitual
experience; for in an immense preponderance of cases at least a visual
or auditory impression does arise through the action on the sense-organ
of a force (ether or air waves) proceeding from a distant object.
In some circumstances, however, even this element of practical truth
disappears, and the localization of the impression, both within and
without the organism, becomes altogether illusory. This result is
involved in the illusions, already spoken of, which arise from the
instinctive tendency to refer sensations to the ordinary kind of
stimulus. Thus, when a feeling resulting from a disturbance in the optic
nerve is interpreted as one of external light vaguely felt to be acting
on the eye, or one resulting from some action set up in the auditory
fibre as a sensation of external sound vaguely felt to be entering the
ear, we see that the error of localization is a consequence of the other
error already characterized.
As I have already observed, an excitation of a nerve at any other point
than the peripheral termination, occurs but rarely in normal life. One
familiar instance is the stimulation of the nerve running to the hand
and fingers, by a sharp blow on the elbow over which it passes. As
everybody knows, this gives rise to a sense of pain at the _extremities_
of the nerve. The most common illustration of such errors of
localization is found in subjective sensations, such as the impression
we sometimes have of something creeping over the skin, of a disagreeable
taste in the mouth, of luminous spots floating across the field of
vision, and so on. The exact physiological seat of these is often a
matter of conjecture only; yet it may safely be said that in many
instances the nervous excitation originates at some point considerably
short of its peripheral extremity: in which case there occurs the
illusion of referring the impressions to the peripheral sense-organ, and
to an external force acting on this.
The most striking instances of these errors of localization are found in
abnormal circumstances. It is well known that a man who has lost a leg
refers all sensations arising from a stimulation of the truncated fibres
to his lost foot, and in some cases has even to convince himself of the
non-existence of his lost member by sight or touch. Patients often
describe these experiences in very odd language. "If," says one of Dr.
Weir Mitchell's patients, "I should say I am more sure of the leg which
ain't than the one which air, I guess I should be about correct."[30]
There is good reason for supposing that this source of error plays a
prominent part in the illusions of the insane. Diseased centres may be
accompanied by disordered peripheral structures, and so subjective
sensation may frequently be the starting-point of the wildest illusions.
Thus, a patient's horror of poison may have its first origin in some
subjective gustatory sensation. Similarly, subjective tactual sensations
may give rise to gross illusions, as when a patient "feels" his body
attacked by foul and destructive creatures.
It may be well to remark that this mistaken interpretation of the seat
or origin of subjective sensation is closely related to hallucination.
In so far as the error involves the ascription of the sensation to a
force external to the sense-organ, this part of the mental process must,
when there is no such force present, be viewed as hallucinatory. Thus,
the feeling of something creeping over the skin is an hallucination in
the sense that it implies the idea of an object external to the skin.
Similarly, the projection of an ocular impression due to retinal
disturbance into the external field of vision, may rightly be named an
hallucination. But the case is not always so clear as this. Thus, for
example, when a gustatory sensation is the result of an altered
condition of the saliva, it may be said that the error is as much an
illusion as an hallucination.[31]
In a wide sense, again, all errors connected with those subjective
sensations which arise from a stimulation of the peripheral regions of
the nerve may be called illusions rather than hallucinations. Or, if
they must be called hallucinations, they may be distinguished as
"peripheral" from those "central" hallucinations which arise through an
internal automatic excitation of the sensory centre. It is plain from
this that the region of subjective sensation is an ambiguous region,
where illusion and hallucination mix and become confused. To this point
I shall have occasion to return by-and-by.
I have now probably said enough respecting the illusions that arise
through the fact of there being fixed limits to our sensibility. The
_rationale_ of these illusions is that whenever the limit is reached, we
tend to ignore it and to interpret the impression in the customary way.
_Variations of Sensibility._
We will now pass to a number of illusions which depend on something
variable in the condition of our sensibility, or some more or less
exceptional organic circumstance. These variations may be momentary and
transient or comparatively permanent. The illusion arises in each case
from our ignoring the variation, and treating a given sensation under
all circumstances as answering to one objective cause.
First of all, the variation of organic state may affect our mental
representation of the strength of the stimulus or external cause. Here
the fluctuation may be a temporary or a permanent one. The first case is
illustrated in the familiar example of taking a room to be brighter
than it is when emerging from a dark one. Another striking example is
that of our sense of the temperature of objects, which is known to be
strictly relative to a previous sensation, or more correctly to the
momentary condition of the organ. Yet, though every intelligent person
knows this, the deeply rooted habit of making sensation the measure of
objective quality asserts its sway, and frequently leads us into
illusion. The well-known experiment of first plunging one hand in cold
water, the other in hot, and then dipping them both in tepid, is a
startling example of this organized tendency. For here we are strongly
disposed to accept the palpable contradiction that the same water is at
once warm and cool.
Far more important than these temporary fluctuations of sensibility are
the permanent alterations. Excessive fatigue, want of proper nutrition,
and certain poisons are well known to be causes of such changes. They
appear most commonly under two forms, exalted sensibility, or
hyperaesthesia, and depressed sensibility, or anaesthesia. In these
conditions flagrant errors are made as to the real magnitude of the
causes of the sensations. These variations may occur in normal life to
some extent. In fairly good health we experience at times strange
exaltations of tactual sensibility, so that a very slight stimulus, such
as the contact of the bed-clothes, becomes greatly exaggerated.
In diseased states of the nervous system these variations of sensibility
become much more striking. The patient who has hyperaesthesia fears to
touch a perfectly smooth surface, or he takes a knock at the door to be
a clap of thunder. The hypochondriac may, through an increase of
organic sensibility, translate organic sensations as the effect of some
living creature gnawing at his vitals. Again, states of anaesthesia lead
to odd illusions among the insane. The common supposition that the body
is dead, or made of wood or of glass, is clearly referable in part to
lowered sensibility of the organism.[32]
It is worth adding, perhaps, that these variations in sensibility give
rise not only to sensory but also to motor illusions. To take a homely
instance, the last miles of a long walk seem much longer than the first,
not only because the sense of fatigue leading us to dwell on the
transition of time tends to magnify the apparent duration, but because
the fatigued muscles and connected nerves yield a new set of sensations
which constitute an exaggerated standard of measurement. A number of
optical illusions illustrate the same thing. Our visual sense of
direction is determined in part by the feelings accompanying the action
of the ocular muscles, and so is closely connected with the perception
of movement, which has already been touched on. If an ocular muscle is
partially paralyzed it takes a much greater "effort" to effect a given
extent of movement than when the muscle is sound. Hence any movement
performed by the eye seems exaggerated. Hence, too, in this condition
objects are seen in a wrong direction; for the patient reasons that they
are where they would seem to be if he had executed a wider movement than
he really has. This may easily be proved by asking him to try to seize
the object with, his hand. The effect is exaggerated when complete
paralysis sets in, and no actual movement occurs in obedience to the
impulse from within.[33]
Variations in the condition of the nerve affect not only the degree, but
also the quality of the sensation, and this fact gives rise to a new
kind of illusion. The curious phenomena of colour-contrast illustrate
momentary alterations of sensibility. When, after looking at a green
colour for a time, I turn my eye to a grey surface and see this of the
complementary rose-red hue, the effect is supposed to be due to a
temporary fatigue of the retina in relation to those ingredients of the
total light in the second case which answer to the partial light in the
first (the green rays).[34]
These momentary modifications of sensibility are of no practical
significance, being almost instantly corrected. Other modifications are
more permanent. It was found by Himly that when the retina is
overexcitable every stimulus is raised in the spectrum scale of colours.
Thus, violet becomes red. An exactly opposite effect is observed when
the retina is torpid.[35] Certain poisons are known to affect the
quality of the colour-impression. Thus, santonin, when taken in any
quantity, makes all colourless objects look yellow. Severe pathological
disturbances are known to involve, in addition to hyperaesthesia and
anaesthesia, what, has been called paraesthesia, that is to say, that
condition in which the quality of sensation is greatly changed. Thus,
for example, to one in this state all food appears to have a metallic
taste, and so on.
If we now glance back at the various groups of illusions just
illustrated, we find that they all have this feature in common: they
depend on the general mental law that when we have to do with the
unfrequent, the unimportant, and therefore unattended to, and the
exceptional, we employ the ordinary, the familiar, and the well-known as
our standard. Thus, whether we are dealing with sensations that fall
below the ordinary limits of our mental experience, or with those which
arise in some exceptional state of the organism, we carry the habits
formed in the much wider region of average every-day perception with us.
In a word, illusion in these cases always arises through what may,
figuratively at least, be described as the application of a rule, valid
for the majority of cases, to an exceptional case.
In the varieties of illusion just considered, the circumstance that
gives the peculiarity to the case thus wrongly interpreted has been
referred to the organism. In the illusions to which we now pass, it will
be referred to the environment. At the same time, it is plain that there
is no very sharp distinction between the two classes. Thus, the visual
illusion produced by pressing the eyeball might be regarded not only as
the result of the organic law of the "specific energy" of the nerves,
but, with almost equal appropriateness, as the consequence of an
exceptional state of things in the environment, namely, the pressure of
a body on the retina. As I have already observed, the classification
here adopted is to be viewed simply as a rough expedient for securing
something like a systematic review of the phenomena.
CHAPTER V.
ILLUSIONS OF PERCEPTION--_continued_.
A. _Passive Illusions (b) as determined by the Environment._
In the following groups of illusion we may look away from |
