Fact and Fable in Psychology

by Joseph Jastrow

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.