What I think is clearly untenable is the view, sometimes urged by Dr. Watson, that when we are, as we think, seeing imaginary pictures with the eyes shut, we are really only using such words as would describe them. It seems to me as certain as anything can be that, when I visualise, something is happening which is connected with the sense of sight. For example, I can call up quite clear mental pictures of the house in which I lived as a child; if I am asked a question as to the furniture of any of the rooms in that house, I can answer it by first calling up an image and then looking to see what the answer is, just as I should look to see in an actual room. It is quite clear to me that the picture comes first and that words after; moreover, the words need not come at all. I cannot tell what is happening in my retina or optic nerve at these moments of visualisation, but I am quite sure that something is happening which has a connection with the sense of sight that it does not have with other senses. And I can say the same of aural and tactual images. If this belief were inconsistent with anything else that seems to me equally certain, I might be induced to abandon it. But so far as I can see, there is no such inconsistency.
It will be remembered that we decided in favour of perceptions as events distinct from those which they perceive, and only causally connected with them. There is, therefore, no reason why association should not work in this region as well as in the region of muscles and glands; in other words, there is no reason to deny what used to be called “association of ideas”, in spite of the fact that bodily changes can also be associated. If a physical basis is wanted, it can be assumed to exist in the brain. The state of the brain which causes us to hear the word “Napoleon” may become associated with the state of brain which causes us to see a picture of Napoleon, and thus the word and the picture will call each other up. The association may be in the sense-organs or nerves, but may equally well be in the brain. So far as I know, there is no conclusive evidence either way, nor even that the association is not purely “mental.”
When we try to find a definition of the difference between a sensation and an image, it is natural to look first for intrinsic differences. But intrinsic differences between ordinary sensations and ordinary images, for example as to “liveliness”, are found to be subject to exceptions, and therefore unsuitable for purposes of definition. Thus we are brought to differences as to causes and effects.
It is obvious that, in an ordinary case, you perceive a table because (in some sense) the table is there. That is to say, there is a causal chain leading backwards from your perception to something outside your body. This alone, however, is hardly sufficient as a criterion. Suppose you smell peat smoke and think of Ireland, your thought can equally be traced to a cause outside your body. The only real difference is that the outside cause (peat smoke) would not have had the effect (images of Ireland) upon every normal person, but only upon such as had smelt peat smoke in Ireland, and not all of them. That is to say, the normal cerebral apparatus does not cause the given stimulus to produce the given effect except where certain previous experiences have occurred. This is a very vital distinction. Part of what occurs in us under the influence of a stimulus from without depends upon past experience; part does not. The former part includes images, the latter consists of pure sensations. This, however, as we shall see later, is inadequate as a definition.
Mental occurrences which depend upon past experience are called “mnemic” occurrences, following Semon. Images are thus to be included among mnemic occurrences, at least so far as human experience goes. This, however, does not suffice to define them, since there are others, e.g. recollections. What further defines them is their similarity to sensations. This only applies strictly to simple images; complex ones may occur without a prototype, though all their parts will have prototypes among sensations. Such, at least, is Hume’s principle, and on the whole it seems to be true. It must not, however, be pressed beyond a point. As a rule, an image is more or less vague, and has a number of similar sensations as its prototypes. This does not prevent the connection with sensation in general, but makes it a connection with a number of sensations, not with one only.
It happens that, when a complex of sensations has occurred at some time in a person’s experience, the recurrence of part of the whole tends to produce images of the remaining parts or some of them. This is association, and has much to do with memory.
It is common to speak of images as “centrally excited”, as opposed to sensations, which are excited by a stimulus to some sense organ. In essence this is quite correct, but there is need of some caution in interpreting the phrase. Sensations also have proximate causes in the brain; images also may be due to some excitement of a sense-organ, when they are roused by a sensation through association. But in such cases there is nothing to explain their occurrence except the past experience and its effect on the brain. They will not be aroused by the same stimulus in a person with similar sense-organs but different past experience. The connection with past experience is clearly known; it is, however, an explanatory hypothesis, not directly verifiable in the present state of knowledge, to suppose that this connection works through an effect of the past experience on the brain. This hypothesis must be regarded as doubtful, but it will save circumlocution to adopt it. I shall therefore not repeat, on each occasion, that we cannot feel sure it is true. In general, where the causal connection with past experience is obvious, we call an occurrence “mnemic”, without implying this or that hypothesis as to the explanation of mnemic phenomena.
It is perhaps worth while to ask how we know that images are like the sensations which are their prototypes. The difficulty of this question arises as follows. Suppose you call up an image of the Brooklyn Bridge, and you are convinced that it is like what you see when you look at Brooklyn Bridge. It would seem natural to say that you know the likeness because you remember Brooklyn Bridge. But remembering is often held to involve, as an essential element, the occurrence of an image which is regarded as referring to a prototype. Unless you can remember without images, it is difficult to see how you can be sure that images resemble prototypes. I think that in fact you cannot be sure, unless you can find some indirect means of comparison. You might, for example, have photographs of Brooklyn Bridge taken from a given place on two different days, and find them indistinguishable, showing that Brooklyn Bridge has not changed in the interval. You might see Brooklyn Bridge on the first of these days, remember it on the second, and immediately afterwards look at it. In looking at it, you might find every detail coming to you with a feeling of expectedness, or you might find some details coming with a feeling of surprise. In this case you would say that your image had been wrong as regards the details which were surprising. Or, again, you might make a picture of Brooklyn Bridge on paper, from memory, and then compare it with the original or a photograph. Or you might content yourself by writing down a description of it in words, and verifying its accuracy by direct observation. Innumerable methods of this kind can be devised by which you can test the likeness of an image to its prototype. The result is that there is often a great likeness, though seldom complete accuracy. The belief in the likeness of an image to its prototype is, of course, not generated in this way, but only tested. The belief exists prior to evidence as to its correctness, like most of our beliefs. I shall have more to say on this subject in the next chapter, which will be concerned with memory. But I think enough has been said to show that it is not unreasonable to regard images as having a greater or less degree of resemblance to their prototypes. To claim more is hardly justifiable.
We can now reach a definite conclusion about perception, sensation, and images. Let us imagine a number of people placed, as far as possible, in the same environment; we will suppose that they sit successively in a certain chair in a dark room, in full view of illuminated pictures of two eminent politicians of opposite parties whose names are written underneath them. We will suppose that all of them have normal eyesight. Their reactions will be partly similar, partly different. If any of these observers are babies too young to have learnt to focus, they will not see sharp outlines, but a mere blurr, not from an optical defect, but from a lack of cerebral control over muscles. In this respect, experience has an effect even upon what must count as pure sensation. But this difference is really analogous to the difference between having one’s eyes open and having them shut; the difference is in the sense-organ, although it may be due to a difference in the brain. We will therefore assume that all the spectators know how to adjust the eyes so as to see as well as possible, and all try to see. We shall then say that, if the spectators differ as widely as is possible for normal human beings, what is common to the reactions of all of them is sensation, provided it is connected with the sense of sight, or, more correctly, provided it has that quality which we observe to be common and peculiar to visual objects. But probably all of them, if they are over three months old, will have tactile images while they see the pictures. And if they are more than about a year old, they will interpret them as pictures, which represent three-dimensional objects; before that age, they may see them as coloured patterns, not as representations of faces. Most animals, though not all, are incapable of interpreting pictures as representations. But in an adult human being this interpretation is not deliberate; it has become automatic. It is, I think, mainly a question of tactile images: the images you have in looking at a picture are not those appropriate to a smooth flat surface, but those appropriate to the object represented. If the object represented is a large one, there will also be images of movement—walking round the object, or climbing up it, or what not. All these are obviously a product of experience, and therefore do not count as part of the sensation. This influence of experience is still more obvious when it comes to reading the names of the politicians, considering whether they are good likenesses, and feeling what a fine fellow one of them looks and what unmitigated villainy is stamped upon the features of the other. None of this counts as sensation, yet it is part of a man’s spontaneous reaction to an outside stimulus.
It is evidently difficult to avoid a certain artificiality in distinguishing between the effects of experience and the rest in a man’s reaction to a stimulus. Perhaps we could tackle the matter in a slightly different way. We can distinguish stimuli of different sorts: to the eye, the ear, the nose, the palate, etc. We can also distinguish elements of different sorts in the reaction: visual elements, auditory elements, etc. The latter are defined, not by the stimulus, but by their intrinsic quality. A visual sensation and a visual image have a common quality which neither shares with an auditory sensation or an auditory image. We may then say: a visual image is an occurrence having the visual quality but not due to a stimulus to the eye, i.e. not having as a direct causal antecedent the incidence of light-waves upon the retina. Similarly an auditory image will be an occurrence having the auditory quality but not due to sound-waves reaching the ear, and so on for the other senses. This means a complete abandonment of the attempt to distinguish psychologically between sensations and images; the distinction becomes solely one as to physical antecedents. It is true that we can and do arrive at the distinction without scientific physics, because we find that certain elements in our integral reactions have the correlations that make us regard them as corresponding to something external while others do not—correlations both with the experience of others and with our own past and future experiences. But when we refine upon this common-sense distinction and try to make it precise, it becomes the distinction in terms of physics as stated just now.