The saying that appearances are deceptive is an inheritance from
ancient times; to Oriental and to Greek philosophers the illusory
nature of the knowledge furnished by the senses was a frequent and
a fertile theme of contemplation and discussion. The same problem
stands open to the psychologist of to-day; but, profiting by the
specialization of learning and the advance of technical science, he
can give it a more comprehensive as well as a more practical answer.
The physiological activities underlying sense-perception are now
fairly well understood; the experimental method has extended its
domain over the field of mental phenomena; and in many ways have we
become more expert in addressing our queries to the sphinx, Nature,
so as to force a reply. To outline the position of modern psychology
with reference to this interesting problem of deception is the object
of the present essay.
I
In a sense-impression we recognize a primitive element in the
acquisition of knowledge. The deprivation of a sense even under most
favorable circumstances leaves some traces of an incomplete mental
development. This is due, not to the mere sense-impressions that the
organ furnishes, but to the perception and coördination of these
by inferential processes of a more complex nature. It is not the
eye of the eagle, but the brain directing the human eye, that leads
to intellectual supremacy. Physiology recognizes this distinction
as one between lower and higher brain-centres. A man may have his
retina or his optic nerve injured, and so be blind in the ordinary
sense of the word. He is prevented from acquiring further knowledge
by this avenue; but, unless he become blind in early childhood, he
will retain a memory for visual images, will be able more or less
clearly to imagine pictorially the appearances of objects from verbal
descriptions, and in the free roamings of his dream fancy will live
in a world in which blindness is unknown. On the other hand, there
is a condition resulting from the disintegration of certain portions
of the finely organized cortex of the brain, in which the patient
may retain full sight and understanding, but be unable to derive
any meaning from what he sees. The same cluster of sensations that
enables us to recognize a book, a picture, a face, and to arouse all
the numerous associations attaching to these, is as unmeaning to
him as the symbols of a cipher alphabet. This condition is termed
"psychic blindness;" and what is there lost is not the power of
vision, but of interpreting, of assimilating, of reading the meaning
of visual sense-impressions. It is this interpreting and assimilating
process that is largely concerned in the formation of illusions.
In the experiences of daily life we have to do not with simple
sensations, but with more or less complex inferences from them;
and it is just because these inferences go on so constantly and so
unconsciously that they are continually and persistently overlooked.
It is an occasional experience in raising a water-pitcher to have
the vessel fly up in the hand in a very startling manner,--the
reason for this being that the pitcher, which one is accustomed to
find well filled, happens to be empty. This experience shows that
we unconsciously estimate the force necessary to raise the vessel,
but only become conscious of this train of inference when it happens
to lead to conclusions contradictory of the fact. The perception
of distance, once thought to be as primitive a factor in cognition
as the impression of a color, is likewise the result of complex
though unrealized inferences; and the phenomena of the stereoscope,
by imitating the conditions of the perception of solidity and thus
making us see as solid the flat representations of a pair of diagrams
or photographs, furnish a brilliant illustration of the variety
and complexity of such unconscious reasonings. As for essential
purposes normal persons have a common anatomy, a common physiology,
and a common psychology, it results that we draw these unconscious
inferences after the same pattern; and so completely are they the
outcome of the normal reactions to our common environment, that
we need not be, and as a rule are not, aware of their existence
until--and probably with some little effort--our attention is
directed to them. Unconsciously and spontaneously we learn to
see,--that is, to extract meaning out of the sense-impressions that
fall upon the retina.
The simplest type of a deception occurs when an inference or an
interpretation of this type, owing to an _unusual disposition of
external circumstances_, leads to a conclusion which other and
presumably superior testimony shows to be false. Thus, in the
observation which Aristotle knew and described, that a pea or other
round object held between two fingers crossed one over the other
seems double, it is the unusual position of the fingers that induces
the illusion. Under ordinary circumstances a sensation of contact
on the left side of one finger and on the right side of the finger
next to it (to the right) could only be produced by the simultaneous
application of two bodies. We unconsciously and naturally make
the same inference when the fingers are crossed, and thus fall
into error--an error, it is important to observe, which we do not
_outgrow_ but _antagonize_ by more convincing evidence. The pea held
between the crossed fingers continues to _feel_ like two peas, but we
are under no temptation to _believe_ that there really are two peas.
The limitations of our senses lead directly to the possibilities
of their deception, which may in turn be realized inadvertently
or utilized intentionally. We appreciate how defective is our
localization of sound when we attempt to find a cricket by locating
whence proceeds its chirp; the same difficulty lends uncertainty
to the determination of the direction of fog-horn signals of
passing steamers. This uncertainty coöperates in the illusions of
ventriloquism; it is involved in the smack which one clown gives
another, but which is really the clapping of the hands of the
supposed victim; it produces a realistic effect when a cannon is
fired on the stage, for it is necessary only to show the flash while
the noise is produced behind the scenes. Again, the stimulation of
the retina is ordinarily due to the impinging upon it of light-waves
emanating from an external object. Accordingly, when the retina is
disturbed by any exceptional cause, such as a blow on the head or
an electric shock to the optic nerve, we have a sensation of light
projected outward into space. The perception of our own locomotion,
which is likewise a highly inferential process, offers illustrations
of casual illusion and of artificial deception. When on a train, it
is by the passing-by before our eyes in the opposite direction of
trees and posts and other features of the landscape, that we realize
that we are moving forward; accordingly, when a train alongside moves
out before our own train has started, we have a distinct realization
that we are moving backwards so long as we look at the forward-moving
train. There is an illusion devised for amusement called the "Magic
Swing," in which one is apparently swung to and fro with wider and
wider excursions until a complete revolution is apparently made
from a vertical to a horizontal, through the antipodal position,
back to the horizontal and the normal. In reality, only a slight
motion is imparted to the swing, but the inclosing walls, which
are painted to represent a forest scene, are themselves revolved
forward and then backward about the axis from which the swing is
suspended. As, however, we have no experience with oscillating trees,
we unconsciously infer the oscillations to be and feel them in our
own persons. In another application of the same illusion we seem to
be let down into the bowels of the earth; but after a slight actual
descent the car remains stationary while the illuminated sides of
the shaft, which are suitably painted, are moved panorama-like in
an upward direction. In brief, we are creatures of the average; we
are adjusted for the most probable event; our organism has acquired
the habits impressed upon it by the most frequent experiences; and
this has induced an inherent logical necessity to interpret a new
experience by the old, an unfamiliar by the familiar. In describing
illusions of the above type, Mr. Sully aptly says that 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." Illusion arises when the rule thus applied fails to
hold; and whether or not we become cognizant of the illusion depends
upon the ease with which the exceptional character of the particular
instance can be recognized, or the inference to which it leads be
opposed by presumably more reliable evidence.