Chapter 30 The Nature of Images and the Introspective Trap Daniel Dennett

Although few philosophers these days will express outright allegiance to the doctrine of mental imagery, these ghostly snapshots have not yet been completely exorcized from current thinking.

Although the myth of mental imagery is beginning to lose its grip on thinkers in the field, it is still worth a direct examination and critique.¹1 shall restrict the examination to visual perception and mental imagery, since the results obtained there can be applied directly to the other sense modalities. We are less inclined to strike up the little band in the brain for auditory perception than we are to set up the movie screen, so if images can be eliminated, mental noises, smells, feels and tastes will go quietly.

The difficulty with mental images has always been that they are not very much like physical images—paintings and photographs, for example. The concept of a mental image must always be hedged in a variety of ways: mental images are in a different space, do not have dimensions, are subjective, are Intentional, or even, in the end, just quasi-images. Once mental images have been so qualified, in what respects are they like physical images at all? Paintings and photographs are our exemplary images, and if mental images are not like them, our use of the word 'image' is systematically mis­leading, regardless of how well entrenched it is in our ordinary way of speaking.

Let me propose an add test for images. An image is a representation of something, but what sets it aside from other representations is that an image represents something else always in virtue of having at least one quality or characteristic of shape, form or colour in common with what it represents. Images can be in two or three dimensions, can be manufactured or natural, permanent or fleeting, but they must resemble what they represent and not merely represent it by playing a role—symbolic, conventional or functional—in some system. Thus an image of an orange need not be orange (e.g., it could be a black-and-white photograph), but something hard, square and black just cannot be an image of something soft, round and white. It might be intended as a symbol of something soft, round and white, and—given the temper of contemporary art—might even be labelled a portrait of something soft, round and white, but it would not be an image. Now I take the important question about mental images to be: are there elements in perception that represent in virtue of resembling what they represent and hence deserve to be called images?

First let us attack this question from the point of view of a sub-personal account of perception. Consider how images work. It is one thing just to be an image—e.g., a reflection in a pool in the wilderness—and another to function as an image, to be taken as an image, to be used as an image. For an image to work as an image there must be a person (or an analogue of a person) to see or observe it, to recognize or ascertain the qualities in virtue of which it is an image of something. Imagine a fool putting a television camera on his car and connecting it to a small receiver under the bonnet so the engine could 'see where it is going'. The madness in this is that although an image has been provided, no provision has been made for anyone or anything analogous to a perceiver to watch the image. This makes it clear that if an image is to function as an element in perception, it will have to function as the raw material and not the end product, for if we suppose that the product of the perceptual process is an image, we shall have to design a perceiver-analogue to sit in front of the image and yet another to sit in front of the image which is the end product of perception in the perceiver- analogue and so forth ad infinitum.

In fact the last image in the physical process of perception is the image of stimulation on the retina. The process of afferent analysis begins on the surface of the retina and continues up the optic nerve, so that the exact pattern of stimulation on the retina is 'lost' and replaced with information about characteristics of this pattern and eventual­ly about characteristics of the environment.² The particular physiological facts about this neural analysis are not directly relevant to the philosophical problem of images. The nervous system might have transmitted the mosaic of stimulation on the retina deep into the brain and then reconstituted the image there, in the manner of television, but in that case the analysis that must occur as the first step in perception would simply be carried out at a deeper anatomical level. Once perceptual analysis has begun there will indeed be elements of the process that can be said to be representations, but only in virtue of being interrelated parts of an essentially arbitrary system. The differ­ence between a neural representation of a square and that of a circle will no more be a difference in the shape of the neural things, than the difference between the words 'ox' and butterfly' is that one is heavier and uglier than the other. The upshot of this is that there is no room in the sub-personal explanation of the perceptual process, whatever its details, for images. Let us turn then to the personal level account of mental imagery to see if it is as compelling, after all, as we often think.

Shorter, in 'Imagination',³ describes imagining as more like depicting—in words— than like painting a picture. We can, and usually do, imagine things without going into great detail. If I imagine a tall man with a wooden leg I need not also have imagined him as having hair of a certain colour, dressed in any particular clothes, having or not having a hat.

If I write down a description of a person it would be absurd for anyone to say that my description cannot fail to mention whether or not the man is wearing a hat. My description can be as brief and undetailed as I like. Similarly it would be absurd to insist

The Nature of Images and the Introspective Trap 213 that one's imagining someone must go into the question of his wearing a hat. It is one thing to imagine a man wearing a hat, another to imagine him not wearing a hat, a third to imagine his head so obscured you can't tell, and a fourth to imagine him without going into the matter of headgear at all. Imagining is depictional or descriptional, not pictorial, and is bound only by this one rule borrowed from the rules governing sight: it must be from a point of view—I cannot imagine the inside and outside of a bam at once.⁴

A moment's reflection should convince us that it is not just imagining, however, that is like description in this way; all 'mental imagery', including seeing and hallucinating, is descriptional. Consider the film version of War and Peace and Tolstoy's book; the film version goes into immense detail and in one way cannot possibly be faithful to Tolstoy's words, since the 'picture painted' by Tolstoy does not go into the detail the film cannot help but go into (such as the colours of the eyes of each filmed soldier).

As soon as images are abandoned even from the personal level account of perception in favour of a descriptional view of awareness, a number of perennial philosophical puzzles dissolve. Consider the Tiger and his Stripes. I can dream, imagine or see a striped tiger, but must the tiger I experience have a particular number of stripes? If seeing or imagining is having a mental image, then the image of the tiger must—obey­ing the rules of images in general—reveal a definite number of stripes showing, and one should be able to pin this down with such questions as 'more than ten?', 'less than twenty?'. If, however, seeing or imagining has a descriptional character, the questions need have no definite answer. Unlike a snapshot of a tiger, a description of a tiger need not go into the number of stripes at all; 'numerous stripes' may be all the description says. Of course in the case of actually seeing a tiger, it will often be possible to comer the tiger and count his stripes, but then one is counting real tiger stripes, not stripes on a mental image/

Another familiar puzzle is Wittgenstein's duck-rabbit, the drawing that looks now like a duck, now like a rabbit. What can possibly be the difference between seeing it first one way and then the other? The image (on the paper or the retina) does not change, but there can be more than one description of that image. To be aware of it first as a rabbit and then as a duck can be just a matter of the content of the signals crossing the awareness line, and this in turn could depend on some weighting effect occurring in the course of afferent analysis.

Of all the problems that have led philosophers to posit mental imagery, the most tenacious has been the problem of hallucinations, and yet it need hardly be mentioned that there is no problem of hallucinations unless one is thinking of awareness imag- istically. On the sub-personal level, there can be little doubt that hallucinations are caused by abnormal neuronal discharges. Stimulation by electrode of micro-areas on the visual cortex produces specific and repeatable hallucinations.⁶ Having a visual hallud-

nation is then just being aware of the content of a non-veridical visual 'report' caused by such a freak discharge. And where is this report, and what space does it exist in7 It is in the brain and exists in the space taken up by whatever event it is that has this non-veridical content, just as my description of hallucinations takes up a certain amount of space on paper. Since spatiality is irrelevant to descriptions, freak descriptions do not require ghostly spaces to exist in.⁷

The one familiar philosophical example that may seem at first to resist the descrip- Honal view of perception and awareness in favour of the imagistic is the distinction, drawn by Descartes, between imagining and conceiving. We can imagine a pentagon or a hexagon, and imagining one of these is introspectively distinguishable from imag­ining the other, but we cannot imagine a chiliagon (a thousand-sided figure) in a way that is introspectively distinct from imagining a 999-sided figure. We can, however, conceive of a chiliagon (without trying to imagine one) and this experience is perfectly distinct from conceiving of a 999-sided figure. From this it might be tempting to argue that whereas conceiving might well be descriptional and not imagistic, imagining must be imagistic, for our inability to imagine a chiliagon is just like our inability to tell a picture of a chiliagon from the picture of a 999-sided figure. All this shows, however, is that imagining is like seeing, not that imagining is like making pictures. In fact, it shows that imagining is not like making pictures, for I certainly can make a picture of a chiliagon if I have a great deal of patience and very sharp pencils, and when it is done I can tell it from a picture of a 999-sided figure, but this deliberate, constructive activity is unparalleled by anything I can do when I 'frame mental images'. Although I can put together elements to make a mental 'image' the result is always bound by a limitation of seeing: I can only imagine what I could see in a glance; differences below the threshold of discrimination of casual observation cannot be represented in imagination. The dis­tinction between imagining and conceiving is real enough; it is like the distinction between seeing and listening to someone. Conceiving depends on the ability to under­stand words, such as the formula 'regular thousand-sided figure', and what we can describe in words far outstrips what we can see in one gaze.

If seeing is rather like reading a novel at breakneck speed, it is also the case that the novel is written to order at breakneck speed. This allows introspection to lay a trap for us and lead us naturally to the picture theory of seeing. Whenever we examine our own experience of seeing, whenever we set out to discover what we can say about what we are seeing, we find all the details we think of looking for. When we read a novel, questions can come to mind that are not answered in the book, but when we are looking at something, as soon as questions come up they are answered immediately by new information as a result of the inevitable shift in the focus and fixation point of our eyes. The reports of perception are written to order; whatever detail interests us is immedi­ately brought into focus and reported on. When this occurs one is not scanning some stable mental image or sense-datum. One is scanning the outside world—quite literal­ly. One can no more become interested in a part of one's visual experience without bringing the relevant information to the fore than one can run away from one's shadow. For this reason it is tempting to suppose that everything one can know about via the eyes is always 'present to consciousness' in some stable picture.

To sit and introspect one's visual experience for a while is not to examine normal sight. When one does this one is tempted to say that it is all very true that there is only a small, central part of the visual field of which one is aware at any moment, and that to describe the whole scene our eyes, our fixaHon point, and our 'focus of interest' must scan the sensory presentation, but that the parts we are not scanning at any moment persist or remain, as a sort of vague, coloured background. Of this background we are

The Nature of Images and the Introspective Trap 215 only 'semi-aware'. Here, however, introspection runs into trouble, for as s∞n as one becomes interested in what is going on outside the beam of the fixation point one immediately becomes aware of the contents of peripheral signals, and this phenomenon is quite different from the ordinary one. While it is true that one can focus on a spot on the wall and yet direct one's attention to the periphery of one's visual field and come up with reports like There is something blue and book-sized on the table to my right; it is vague and blurred and I am not sure it is a book', it cannot be inferred from this that when one is not doing this one is still aware of the blue, booklike shape. We are led to such conclusions by the natural operation of our eyes, which is to make a cursory scanning of the environment whenever it changes and as soon as it changes, and by the operation of short-term memory, which holds the results of this scanning for a short period of time. In familiar surroundings we do not have to see or pay attention to the objects in their usual places. If anything had been moved or removed we would have noticed, but that does not mean we notice their presence, or even that we had the experience (in any sense) of their presence. We enter a room and we know what objects are in it, because if it is a familiar room we do not notice that anything is missing and thus it is filled with all the objects we have noticed or put there in the past. If it is an unfamiliar room we automatically scan it, picking out the objects that fill it and catch our attention. I may spend an afternoon in a strange room without ever being aware (in any sense) of the colour of the walls, and while it is no doubt true that had the walls been bright red I would have been aware of this, it does not follow that I must have been aware that they were beige, or aware that they were colourless or vaguely coloured—whatever that might mean.⁸

It is true, of course, that when we see we do not simply see that there is a table in front of us, but a table of a particular colour and shape in a particular position and so forth. All this need mean is that the information we receive is vivid and rich in detail. This is not true of the vision of many lower animals. The frog, for example, can see that there is a small moving object before him, but he cannot see that it is a fly or a bit of paper on a string. If the small object is not moving, he cannot see it at all, because motion signals are required for the production of the higher-level signals that will initiate a behavioural response. A frog left in a cage with freshly killed (unmoving) flies will starve to death, because it has no equipment for sending the signal: there is a fly (moving or still). Dangle a dead fly on a string and the frog will eat it.⁹ The difference in degree of complexity and vividness between frog and human perception does not warrant the assumption that there is a difference in kind—however much we may feel that a picture is worth a thousand words.¹⁰

Notes

1. Optimists who doubt that mental images are still taken seriously in philosophy and even in science are invited to peruse two recent anthologies, R. J. Hirst, ed., Perception and the External World, New York, 1965, and J. R. Smythies, ed,, Brain and Mind, Modem Concepts of the Nature of Mind, London, 1965. The wealth of cross-disciplinary confusions over mental images is displayed in both volumes, which both include papers by philosophers, psychologists and neurophysiologists. Neither editor seems to think that much of what he presents is a dead horse, which strengthens my occasionally flagging conviction that I am not beating one. On the other hand there are scientists who have expressed clear and explicit rejections of imagistic confusions. See, e.g., G. W. Zopf, 'Sensory Homeo­stasis' in Wiener and Schad6, Nerve, Brain and Memory Models, New Yoik, 1963, p. 71, esp. p. 118, and D. M. MacKay, 'Internal Representation of the External Worid', unpublished, read at the Avionics Panel Symposium on Nature and Artificial Logic Processors, Athens, July 15-19,1963.

2. H. B. Barlow, Tossible Principles Underiying the Transformations of Sensory Messages' in W. A. Rosenblith, (ed.) Sensory Communication, New Yoric, 1961, offers a particularly insightful account of the 'editorial' function of afferent neural activity and the depletion of information that is the necessary concomitant of such analysis.

3. J. M. Shorter, TmaginaHon', Mind, LXl, 1952, pp. 528-42.

4. Counter-examples spring to mind, but are they really counter-examples? All the ones that have so far occurred to me him out on reflection to be cases of imagining myself seeing—with the aid of large mirrors—the inside and outside of the bam, imagining a (partially) transparent bam, imagining look­ing in the windows and so forth. These are all from a point of view in the sense I mean. A written descripHoα however, is not bound by these IimitaHons; from what point of view is the desαipHon: 'the bam is dark red with black rafters and a pine AooH

5. In the unusual phenomenon of 'eideHc imagery', the subject can read off or count off the details of his 'memory image', and this may seem to provide the fatal counter-example to this view. (See G. Allport, ^zEidetic Imagery', British Journal of Psychology, XV, 1924, pp. 99-120.) Yet the fact that such 'eideHc memory images' actually appear to be projected or superimposed on the subject's normal visual field (so that if the subject shifts his gaze the posiHon of the memory image in his visual field remains fixed, and 'moves with the eye') strongly suggests that in these cases the actual image of retinal StimulaHon is somehow retained at or very near the retina and superimposed on incoming StimulaHon. In these rare cases, then, the memory mechanism must operate prior to afferent analysis, at a time when there still is a physical image.

6. Penfield, The Excitable Cortex in Conscious Man, Liverpool 1958. Some of Penfield's interpretaHons of his results have been widely criHdzed, but the results themselves are remarkable. It would be expected that IialludnaHons would have to be the excepHon rather than the rule in the brain for event—types to acquire content in the first place, and this is in fact supported by evidence. Amputees usually experience 'phantom limb' sensaHons that seem to come from the missing limb; an amputee may feel that he not only still has the leg, but that it is itching or hot or bent at the knee. These phenomena, which occur off and on for years following amputaHoα are nearly universal in amputees, with one interesting excepHon. In cases where the amputaHon occurred in infancy, before the child developed the use and CoordinaHon of the limb, phantom limb is rarely experienced, and in cases where amputa­Hon occurred just after birth, no phantom limb is ever experienced (see M. SimmeL Thantom Experi­ences following AmputaHon in Childhood', Joum. of Neurology, Neurosurgery and Psychiatry XXV, 1962, pp. 69-78).

7. Other phenomena less well known to philosophers also favour a descripHonal explanaHon. See, e.g., W. R. Brain's account of the reports of paHents who have their sight surgically restored, in 'Some ReflecHons on Mind and Brain,' Brain, LXXXVI, 1963, p. 381; the controversial accounts of newly sighted adults' efforts to Ieam to see, in M. von Sendea Raum- und Gestaltauffassung bei Operierten Blindgeborenen υor und nach der Operation, Leipzig, 1932, translated with appendices by P. Heath as Space and Sight, the Perception of Space and Shape in the congenitally blind before and after operation, Londoa I960; I. Kohler's experiments with inverting spectacles (a good account of these and similar experi­ments is found in J. G. Taylor, The Behaoioral Basis of Perception, New Havea 1962); and the disorder called Simultanagnosia, M. Kinsboume and E. K. Warringtoa 'A Disorder of Simultaneous Form PercepHon', Brain, LXXXV, 1962, pp. 461-86 and A. R. Luria, et al., 'Disorders of Ocular Movement in a Case OfSimultanagnosia', Brain, LXXXVI, 1963, pp. 219-28.

8. Cf. Wittgensteia Tut the existence of this feeling of strangeness does not give us a reason for saying that every object we know well and which does not seem strange to us gives us a feeling of familiarity'. Philosophical Investigations, Oxford, 1953, i. 596. See also i. 597, i. 605.

9. Muntz, Vision in Frogs', Scientific American, 210, 1964, pp. 757-76, and Wooldridge, The Machinery of the Brain, New York, 1963, pp. 46-50.

10. Having found no room for images in the sub-personal account of percepHoα we can say that 'mental image' and its kin are poor candidates for referring expressions in science; having found further that nothing with the traits of genuine images is to be found at the personal level either allows us to conclude that 'mental image' is valueless as a referring expression under any circumstances.