We must picture to ourselves, then, in the animal organism, a multitude of nerve-fibres running inwards from all the end-organs of the special senses, from the muscles, and from the internal organs, and all converging on the central nervous system. And we must picture to ourselves a multitude of nerve-fibres passing outwards from the central system, and diverging to supply the muscles, glands, and other organs which are to respond to the stimulation from without. We must picture the fibres coming from or going to related parts or organs collecting together to form nerves and nerve-trunks, which are, however, only bundles of isolated nerve-fibres. And, lastly, we must picture the central nerve-system itself co-ordinating and organizing the stimuli brought into it by afferent nerves, from the organs of special sense, and handing over the resultants by efferent nerves to the organs of special activities. So far we have purely physiological effects, many of which occur with surprising accuracy and precision when an organism is in a state of unconsciousness. Place your finger in the palm of a sleeping child, and the fingers will close over it without the child awaking to consciousness. If, in a frog, the brain of which has been extirpated, the side be touched with a drop of acid, the leg of that side will be drawn up, and the foot will be used to wipe away the acid. And if that leg be held and prevented from reaching the side, the other leg will be brought round so as to try and bring the foot within reach of the irritated spot. The actions are, however, in all probability, purely physiological, and are performed in complete absence of consciousness.
When we turn from the physiological to the psychological aspect of the question, we enter a new world, the world of consciousness, wherein the impressions received by the recipient organs (no longer regarded as mere stimuli, but as the elements of consciousness) are co-ordinated and organized, and are built up into those sensations and perceptions through which the objects of the external world take origin and shape. It is with this process that we have now to deal; and we will deal with it first in man.
The first fact to notice is that, apart from sense-stimuli received and exciting consciousness, we have also the revival of past impressions. This revival is the germ of memory. What exactly is the physical basis of memory, how the effects of stimuli in consciousness come to be registered, we do not know. It is clearly a matter that falls under the general law of persistence; but in what organic manner we are largely ignorant. Still, there can be no question of the fact that, quite apart from impressions due to immediate influences of the environment now acting on our recipient organs, we have also revivals of bygone influences of the environment—shadows or after-images of previous modes of influence. Without this process of registration and revival, stimuli could never give rise to sensations and perceptions such as we know them. Without it experience would be impossible.
We may say, then, that impressions (resulting from stimuli) and their revival in memory are the bricks of the house of knowledge; and these are built up through experience into what we call the world of things around us. There may be and is a certain amount of mortar, supplied by the builder, in addition to the elementary bricks. But without the bricks no house of knowledge could be built. Let us now examine the bricks and the building.
From what we have already learnt in the chapter on "The Senses of Animals," it is clear that the impressions and their revivals in memory have differences in quality. Here, on the very threshold of the subject, we must pause. They have differences of quality. But in consciousness these differences must be distinguished. And this involves their recognition and discrimination, presupposing, therefore, a corresponding faculty, however simple, on the part of the recipient. Without cognition and recognition (twin sisters, born in the same hour) we can never get beyond mere impressions; which may, indeed, be differentiated physically, as different stimuli due to diverse action of the environment, but are psychically undifferentiated. This recognition and discrimination is thus the primary activity of the recipient mind. Here is already some of the mortar supplied by the builder. Memory is absolutely essential to the process. The sense-impression of external origin gives rise to an impression of similarity or dissimilarity, which is part of the internal reaction to the external stimulus. Thus impressions are raised to the level of sensations. A sensation is an impression that has been discriminated from others, and recognized as being of such and such a nature. The impressions of the sense-organs as we know them are thus not mere impressions, but impressions raised to the level of sensations, in so far as they are recognized and discriminated.
Let us now glance at some of the differences in quality recognized in sensation. First, we have the broadly distinguished groups of touches and pressures, temperature-sensations, tastes, smells, sounds, sights, muscular sensations, and organic sensations from internal parts of the body. And then, within each of these groups, there are the more or less delicate and distinct shades of quality, well exemplified in vision by the different colour-sensations, in hearing by notes of different pitch, and in smell by the varieties of scents and odours. Many of those sensations, moreover, which are apparently simple, are in reality compound. There are differences of quality in the note A as sounded on a violin, a piano, and a flute; and these differences are due to different admixtures of overtones, which fuse with the fundamental tone and alter its timbre. So, too, with vision. The sensation given by a white disc is a compound sensation, due to waves of different period, which separately would give sensations of colour. Sensations, then, differ in quality.
They also differ in quantity or intensity. This needs little illustration. As evening falls, the sight-sensations derived from the surrounding objects grow more and more feeble. They may remain the same in quality, but the quantity or intensity gradually diminishes. So, too, in music, the pianos and fortes give us differences in intensity of sound-sensations.
Sensations also differ in duration. The stimulation may be either prolonged or instantaneous. Two or more sensations may, moreover, be simultaneous or successive. Just as they may be either similar or different in quality and in intensity, so they may be either simultaneous or successive in time. Simultaneous sensations are best exemplified in vision and through touch; successive sensations are given most clearly by the sense of hearing, through which we recognize a sequence of sounds.
And then, again, sensations not only differ in time, but they seem also to differ in place. A sensation of touch may be referred to different parts of the body—the hand, the foot, or the forehead. But here we open up an important question—Where do we feel a sensation, such as, for example, that of pressure on the skin? Common sense answers, without hesitation, that we feel it at the particular part of the body which is affected by the external stimulus. I feel the pen with which I write with my finger-tips. And common sense is perfectly right from its own point of view. But it is a well-known fact that a person whose leg has been amputated experiences at times tickling and uneasiness in the absent member. This is due to irritation of the nerve-ends in the stump of the limb. But the sensations are referred outwards to the normal source of origin of impressions, the effects of which were carried inwards by the nerve affected. We shall have to consider hereafter the nature of the relation between physiological and psychological processes—the connection of mind and body. Assuming for the present that psychical processes have a physical basis in physiological processes, the fact given above and others of like implication seem to show that the sensation has for its physiological basis some nerve-change in the central nervous system—in us, no doubt, in the brain. Of course, it must be remembered that the sensation, as felt, is a mental fact (using the word "mental" in its broadest sense, as belonging to the psychical as opposed to the physiological series). But it would seem that the physiological accompaniment of this mental fact is some nerve-change in the brain. This nerve-change is caused by a stimulus having its origin in the end-organ of the afferent nerve, and we naturally refer the impression outwards to the place of its source of origin under ordinary and normal conditions. In other words, we localize it. That is what common sense means when it says that we feel pressure at the finger-tips.