We might therefore conclude that an image is an occurrence having the quality associated with stimulation by some sense-organ, but not due to such stimulation. In human beings, images seem to depend upon past experience, but perhaps in more instinctive animals they are partly due to innate cerebral mechanisms. In any case dependence upon experience is not the mark by which they are to be defined. This shows how intimate is the dependence of traditional psychology upon physics, and how difficult it is to make psychology into an autonomous science.
There is, however, still a further refinement necessary. Whatever is included under our present definition is an image, but some things not included are also images. The sight of an object may bring with it a visual image of some other object frequently associated with it. This latter is called an image, not a sensation, because, though also visual, it is not appropriate to the stimulus in a certain sense: it would not appear in a photograph of the scene, or in a photograph of the retina. Thus we are forced to say: the sensation element in the reaction to a stimulus is that part which enables you to draw inferences as to the nature of the extra-cerebral event (if any) which was the stimulus;[9] the rest is images. Fortunately, images and sensations usually differ in intrinsic quality; this makes it possible to get an approximate idea of the external world by using the usual intrinsic differences, and to correct it afterwards by means of the strict causal definition. But evidently the matter is difficult and complicated, depending upon physics and physiology, not upon pure psychology. This is the main thing to be realised about images.
[9] I.e. the immediate stimulus, not the “physical object”.
The above discussion has suggested a definition of the word “image”. We might have called an event an “image” when it is recognisably of the same kind as a “percept”, but does not have the stimulus which it would have if it were a percept. But if this definition is to be made satisfactory, it will be necessary to substitute a different word in place of “percept”. For example, in the percept of a visible object it would be usual to include certain associated tactual elements, but these must, from our point of view, count as images. It will be better to say, therefore, that an “image” is an occurrence recognisably visual (or auditory, etc., as the case may be), but not caused by a stimulus which is of the nature of light (or sound etc., as the case may be), or at any rate only indirectly so caused as a result of association. With this definition, I do not myself feel any doubt as to the existence of images. It is clear that they constitute most of the material of dreams and day-dreams, that they are utilised by composers in making music, that we employ them when we get out of a familiar room in the dark (though here the rats in mazes make a different explanation possible), and that they account for the shock of surprise we have when we take salt thinking it is sugar or (as happened to me recently) vinegar thinking it is coffee. The question of the causation of images—i.e. whether it is in the brain or in other parts of the body—is not one which it is necessary to our purposes to decide, which is fortunate, since, so far as I know, there is not at present any adequate evidence on the point. But the existence of images and their resemblance to perception is important, as we shall see in the next chapter.
Images come in various ways, and play various parts. There are those that come as accretions to a case of sensation, which are not recognised as images except by the psychologist; these form, for example, the tactual quality of things we only see, and the visual quality of things we only touch. I think dreams belong, in part, to this class of images: some dreams result from misinterpreting some ordinary stimulus, and in these cases the images are those suggested by a sensation, but suggested more uncritically than if we were awake. Then there are images which are not attached to a present reality, but to one which we locate in the past; these are present in memory, not necessarily always, but sometimes. Then there are images not attached to reality at all so far as our feeling about them goes: images which merely float into our heads in reverie or in passionate desire. And finally there are images which are called up voluntarily, for example, in considering how to decorate a room. This last kind has its importance, but I shall say nothing more about it at present, since we cannot profitably discuss it until we have decided what we are to mean by the word “voluntary”. The first kind, which comes as an accretion to sensation, and gives to our feeling of objects a certain rotundity and full-bloodedness which the stimulus alone would hardly warrant, has been considered already. Therefore what remains for the present is the use of images in memory and imagination; and of these two I shall begin with memory.
[CHAPTER XVIII]
IMAGINATION AND MEMORY
In this chapter we have to consider the two topics of imagination and memory. The latter has already been considered in [Chapter VI], but there we viewed it from outside. We want now to ask ourselves whether there is anything further to be known about it by taking account of what is only perceptible to the person remembering.
As regards the part played by images, I do not think this is essential. Sometimes there are memory-images, sometimes not; sometimes when images come in connection with memory, we may nevertheless know that the images are incorrect, showing that we have also some other and more reliable source of memory. Memory may depend upon images, as in the case mentioned above, of the house where I lived as a child. But it may also be purely verbal. I am a poor visualiser, except for things I saw before I was ten years old; when now I meet a man and wish to remember his appearance, I find that the only way is to describe him in words while I am seeing him, and then remember the words. I say to myself: “This man has blue eyes and a brown beard and a small nose; he is short, with a rounded back and sloping shoulders”. I can remember these words for months, and recognise the man by means of them, unless two men having these characteristics are present at once. In this respect, a visualiser would have the advantage of me. Nevertheless, if I had made my verbal inventory sufficiently extensive and precise, it would have been pretty sure to answer its purpose. I do not think there is anything in memory that absolutely demands images as opposed to words. Whether the words we use in “thought” are themselves sometimes images of words, or are always incipient movements (as Watson contends), is a further question, as to which I offer no opinion, since it ought to be capable of being decided experimentally.
The most important point about memory is one which has nothing to do with images, and is not mentioned in Watson’s brief discussion. I mean the reference to the past. This reference to the past is not involved in mere habit memory, e.g. in skating or in repeating a poem formerly learned. But it is involved in recollection of a past incident. We do not, in this case, merely repeat what we did before: then, we felt the incident as present, but now we feel it as past. This is shown in the use of the past tense. We say to ourselves at the time “I am having a good dinner”, but next day we say “I did have a good dinner”. Thus we do not, like a rat in a maze, repeat our previous performance: we alter the verbal formula. Why do we do so? What constitutes this reference of a recollection to the past?[10]
[10] On this subject, cf. Broad, The Mind and Its Place in Nature, p. 264 ff., in his chapter on “Memory”.