Psychology: an elementary text-book

by Hermann Ebbinghaus

[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.