_A. THE INTELLECT_
§ 14. PERCEPTION
1. _Characteristics of Perception_
At every moment of waking life a multitude of impressions are received by the mind through the eyes, the ears, the cutaneous and all other senses, giving information about processes in the external world and in the subject’s own body. However, because of the peculiar laws of mental activity, the actual conscious experience differs greatly from a mere sum of all those impressions--from what would be the content of consciousness if mind were nothing but an accumulation of senses. In order to distinguish the actual consciousness from the abstractly conceived sum of sensations, we use as a specific term the word _perception_.
Does not a newspaper look different if held in the right way or turned upside down, a landscape if seen in the ordinary way or through our legs? In the latter case there are in our consciousness a multitude of incomprehensible details, lines, figures, colors; in the former we are conscious of one thing, a landscape, with its divisions, each of these divisions with its subdivisions, and so on. The one consciousness is practically the result only of simultaneous sensory stimulations; the other consciousness, in addition to these stimulations, is determined by the laws of organized mind, by attention, memory, practice.
A percept contains both less and more than the sensations corresponding directly to the stimulations. According to the conditions discussed under attention, certain sensations become focal at the expense of others which become marginal. For example, of all things impressing themselves upon my retina, only a few--usually, but not always, those in the center of the field of vision--attain a high degree of consciousness. And of these things again not all the qualities, but only a few become highly conscious. If, as in this case, the visible things happen to become highly conscious, the simultaneously existing audible or tastable things are apt to remain at a low degree of consciousness. That which is important for the needs of our daily life is specially favored and becomes a part of the percept. That which has no practical importance does not easily become a highly conscious part of the present mind. The variations in color of a gown forming many folds are rarely noticed. All parts of the gown are perceived as parts of the same substance. That the whole gown is made of one kind of cloth is practically important. That the various folds appear to the eye--because of the variation of the illumination--somewhat different, is of no practical consequence. Many quite common phenomena, after-images, overtones, difference tones, are never known by the majority of people, because of their practical unimportance.
But a percept contains not only less, but also much more than the sensations corresponding to the stimuli of the moment. Numerous images are woven into this system of sensations and thus give additional meaning to it. We may be said to _see_ that the things are hot or cold, rough or smooth, heavy or light, although our eyes as mere sense organs cannot give us any such information. In the same way we may be said to see that the things are at this or that distance from our head, and that this thing is nearer, that thing farther from us, although our inherited ability to see things spatially does not give us any other information than that of shape and size in the field of vision. By incessantly repeated experiences we have learned, at an early age, that changes in the distance of things which in this or that way have come to our knowledge, are regularly accompanied by definite changes in their size, their coloring, their appearance when the right eye’s image is compared with the left eye’s image, and many similar changes of the impression. Whenever such signs of changes in the distance are impressed upon our mind, we immediately supplement them by ideas of the distances themselves. Thus our original two-dimensional perception of space is expanded into a three-dimensional perception.
All knowledge of things, of their properties, their names, their uses, their meanings, consists in supplementing our consciousness of those qualities which they present to our senses, by images previously obtained through any senses. The force of this supplementing can be understood from the drawings of children and primitive peoples. That which appears in the field of vision is often left unrepresented. Linear perspective, for instance, does not exist in such drawings, although it is a part of the sensory impression. On the other hand, many things are given by the draughtsman which are invisible under the circumstances of the situation, but which he regards as essential parts of the thing because of their practical importance: for instance, both eyes of a person seen in profile, equal length of all the legs of tables and chairs, equal size of things at a distance and things near by.
The significance of this supplementing by ideas is illustrated also in pathological cases. It happens that some of the associative connections in the brain are destroyed by disease, reducing the mind to a condition like that of early childhood, when direct sense impressions alone determined action. Patients may see the shape and color of a thing correctly, may even be able to draw it or paint it, but are unable to tell the name of the object, although they are perfectly familiar with it. They cannot answer our question as to what purpose the thing serves; possibly they give ridiculous answers, fitting an altogether different thing. Only when they are permitted to use the kinesthetic and tactual senses by taking the thing in their hands, do they recognize it. In other cases the patient, although possessing his normal sensibility to touch, is unable to recognize things by his hands alone, but recognizes them at once when permitted to open his eyes.
A particularly characteristic feature of our perception is the grouping together into a mental unit of elements which are not united either spatially by contiguity or nearness, or by similarity of their coloring, or their other attributes. The grouping of such elements into a unitary mental state is often the result of a repeated necessity for reacting upon this sum of impressions by a unitary movement. The newspaper held upside down does not invite the reaction of reading. Parts which are separated by blank spaces or by black bars, are separately perceived. But the words and sentences are not perceived, because we have not previously been obliged to read under such conditions. Looking into a furnished room I perceive at once tables, chairs, and other pieces of furniture, although the legs of a chair, for example, are spatially and by their coloring better connected with the carpet than with the back of the chair. When I am looking at a portrait standing upside down, the dark hair and the dark background become a mental unit, a percept of a dark area. The light face is another mental unit. In upright position the hair separates from the background and unites with the face. I then perceive a person before a dark background, in spite of the similarity of coloring between some parts of the figure and the background, in spite of the difference of coloring between some parts of the figure and other parts. The grouping of the elements in perception is therefore widely different from that which would result from the stimuli directly. It is determined by our habits of reaction upon such groups as frequently appear together in the world in which we live.
[Illustration: FIG. 16.--TWO POSSIBILITIES OF PERCEPTION.]
Let us illustrate this by two figures. Figure 16 may be perceived as a rabbit’s or as a duck’s head. When we perceive the figure as a rabbit’s head, the white streaks to the right of the eye are two separate sensation groups, each of them unified with respect to the effect produced by them in our nervous system. They are then the animal’s lips. At the same time the protrusions to the left make us conscious of softness, warmth, flexibility. Now perceive the figure as a duck’s head. Immediately those white streaks cease to be two separate units for our mind. Together with the darker parts surrounding them, they affect our mind as a single unit, the variegated back part of the duck’s head. And at the same time the protrusions to the left make us conscious of hardness, cold, rigidity. The sensory stimulations are exactly the same, but they are differently grouped together, and they bring about further nervous activities which greatly differ in these two perceptions.
[Illustration: FIG. 17.--VARIETIES OF PERCEPTION.]
Figure 17, when shown to a person, is perceived as the result of a child’s careless handling of his ink bottle, as an ink spot. But ask this person if he does not see a boy falling downstairs, and immediately certain elements are grouped together and affect us as being the legs, other elements of sensation are perceived as the arms, and so on. And now suggest to the same person to turn the page slightly to the right and see a man trying to put on his shirt. Quickly the perception changes again; but this time not so much by the breaking up of the former units into their sensory elements and the formation of new units, as by a change of the accompanying ideas. The previous suggestion tends to make us perceive these sensations in one or the other way because it guides our attention. But this guidance is possible only because certain groups of sensational elements (for example, the groups illustrated by our figures) have very often occurred in our mind in consequence of the fact that they originate from external objects which have often been presented to our sense organs among greatly varying surroundings. Thus we have learned to group these elements together and to neglect, more or less, all other elements which may be presented simultaneously.
The total process of selective grouping and of furnishing the groups formed with additional mental contents has often been called _apperception_. But this meaning of the term apperception is not universally adopted. Some mean by apperception mainly the selective grouping of the elements, others mean by it exclusively the furnishing with ideational contents. Because of its ambiguity the term _apperception_ has been entirely omitted from the present book, and the term _perception_ is used in its broadest sense, including both the processes just mentioned. Perception thus means the working over by the mind of any aggregate of sensational elements given at the time through the sense organs.
2. _Illusions_
While the laws of perception are, on the whole, of the greatest benefit to the organism surrounded by a confusing multitude of physical elements bound together into a large number of more or less stable compounds, of things, there are exceptional cases in which these same laws lead the mind into a reaction not suitable to the situation presented.
That which has often occurred is likely to recur. But it does not regularly recur in the same manner. There are exceptions. It happens that certain things occur in surroundings different from their usual surroundings. These things are then perceived, that is, grouped together and supplemented by images, in harmony with their usual surroundings. But the perception is then in discord with the actual surroundings. To the inhabitant of the plains the colors of things appear rather saturated, and the outlines sharp, when these things are at a small distance from the observer. Walking toward them, he is soon able to lay hands on them. But when the air happens to be unusually moist, and because of its diminished weight, free from the particles of dust which have settled because of their weight, things look unusually near, and on walking toward them he discovers that it takes more time to reach them than he expected. The same happens when he goes to the mountains for his vacation, because there the air is always comparatively free from dust. We have here a foreseeing of what ordinarily becomes the subsequent experience, but fails to become it in this instance.
There is another kind of illusion based on the fact that sensations which have been imagined just before the stimuli became effective, are thereby favored and become unusually vivid. This law of attention holds good also when the stimuli are not in exact correspondence with the preceding images. In such a case the perception is more or less assimilated to those images, so that the same stimuli result in somewhat different percepts according to circumstances. “How heavy it is!” said a friend of Davy’s, when the discoverer of potassium placed a little piece of this metal on his finger. Potassium is so light that it floats on water, but the metallic appearance produced the image of pressure and changed the sensation into a percept of something heavy. When two pieces of gray paper, equally bright but of slightly different coloring, are put before me side by side, and I ask myself: is not the yellowish paper lighter than the bluish paper, immediately it seems to be lighter. But I begin to doubt and ask myself: is not the yellowish paper darker than the other; and immediately it looks darker.
Let no one say that this is only “imaginary,” meaning by this word that there are in my mind both the objectively true impression and an incorrect image of something similar. Such is not the case. There is no duality of consciousness. There is one unitary experience. Only scientific reflection reveals the fact that this unitary experience has two sources, one in the external stimulation, the other in the central nervous excitation. The result of these sources, the percept, does not betray the doubleness of its origin any more than a stream at its mouth shows the doubleness of its sources. It is a universal property of perception to be determined not by sensory stimulation alone, although this is the primary factor, but also by images, by nervous dispositions. The more vivid such images, the greater is their influence--now and then their _deceptive_ influence--on our consciousness of the objectively existing. Suggestion is a name which has recently been accepted for such an influence. Illusion is another name for it, in case it is rather pronounced and ill adapted to the object.
