Past Experience Conserved in Both Mental and Physical Terms.—If past experience plays so important a part in our welfare, how, then, is it to be conserved so that we may secure its benefits? Here, as elsewhere, we find the mind and body working in perfect unison and harmony, each doing its part to further the interests of both. The results of our past experience may be read in both our mental and our physical nature.
On the physical side past experience is recorded in modified structure through the law of habit working on the tissues of the body, and particularly on the delicate tissues of the brain and nervous system. This is easily seen in its outward aspects. The stooped shoulders and bent form of the workman tell a tale of physical toil and exposure; the bloodless lips and pale face of the victim of the city sweat shop tell of foul air, long hours, and insufficient food; the rosy cheek and bounding step of childhood speak of fresh air, good food and happy play.
On the mental side past experience is conserved chiefly by means of images, ideas, and concepts. The nature and function of concepts will be discussed in a later chapter. It will now be our purpose to examine the nature of images and ideas, and to note the part they play in the mind's activities.
The Image and the Idea.—To understand the nature of the image, and then of the idea, we may best go back to the percept. You look at a watch which I hold before your eyes and secure a percept of it. Briefly, this is what happens: The light reflected from the yellow object, on striking the retina, results in a nerve current which sets up a certain form of activity in the cells of the visual brain area, and lo! a percept of the watch flashes in your mind.
Now I put the watch in my pocket, so that the stimulus is no longer present to your eye. Then I ask you to think of my watch just as it appeared as you were looking at it; or you may yourself choose to think of it without my suggesting it to you. In either case the cellular activity in the visual area of the cortex is reproduced approximately as it occurred in connection with the percept, and lo! an image of the watch flashes in your mind. An image is thus an approximate copy of a former percept (or several percepts). It is aroused indirectly by means of a nerve current coming by way of some other brain center, instead of directly by the stimulation of a sense organ, as in the case of a percept.
If, instead of seeking a more or less exact mental picture of my watch, you only think of its general meaning and relations, the fact that it is of gold, that it is for the purpose of keeping time, that it was a present to me, that I wear it in my left pocket, you then have an idea of the watch. Our idea of an object is, therefore, the general meaning of relations we ascribe to it. It should be remembered, however, that the terms image and idea are employed rather loosely, and that there is not yet general uniformity among writers in their use.
All Our Past Experience Potentially at Our Command.—Images may in a certain sense take the place of percepts, and we can again experience sights, sounds, tastes, and smells which we have known before, without having the stimuli actually present to the senses. In this way all our past experience is potentially available to the present. All the objects we have seen, it is potentially possible again to see in the mind's eye without being obliged to have the objects before us; all the sounds we have heard, all the tastes and smells and temperatures we have experienced, we may again have presented to our minds in the form of mental images without the various stimuli being present to the end-organs of the senses.
Through images and ideas the total number of objects in our experience is infinitely multiplied; for many of the things we have seen, or heard, or smelled, or tasted, we cannot again have present to the senses, and without this power we would never get them again. And besides this fact, it would be inconvenient to have to go and secure afresh each sensation or percept every time we need to use it in our thought. While habit, then, conserves our past experience on the physical side, the image and the idea do the same thing on the mental side.
3. INDIVIDUAL DIFFERENCES IN IMAGERY
Images to Be Viewed by Introspection.—The remainder of the description of images will be easier to understand, for each of you can know just what is meant in every case by appealing to your own mind. I beg of you not to think that I am presenting something new and strange, a curiosity connected with our thinking which has been discovered by scholars who have delved more deeply into the matter than we can hope to do. Every day—no, more than that, every hour and every moment—these images are flitting through our minds, forming a large part of our stream of consciousness. Let us see whether we can turn our attention within and discover some of our images in their flight. Let us introspect.