CHAPTER 13

The criterion of bodily size and shape

922. When we wish to know if a thing is true or false, we have to compare it with the genuine, certain notion of the same thing. The power we possess of perceiving a thing immediately, rather than its sign or image, is that which gives us this genuine, certain notion or essence.
We have already seen that extension is a mode of the fundamental feeling.(201) Hence the fundamental feeling is a power whose immediate term is not only matter, but also extension. It is the fundamental feeling, therefore, that gives genuine, certain extension, and with it the first measure of every size.

923. But the extension of the fundamental feeling is partly commensurate with extension (cf. 841).
As a result, touch also furnishes the genuine, certain size of bodies and, because of the impossibility of an immediate application of the measure provided by the fundamental feeling, becomes in fact the measure used.

924. On the contrary, the eye and other senses, in so far as they differ from touch: 1. do not perceive immediately the size of distant things; 2. do not perceive their distance, but only signs of distance. The size of things presented by the eye has to be compared and rectified with that given by touch. If sight is not to be the source of error for us, we must continually relate the size we see to that offered by touch. This is the fixed measure provided by nature for comparison and emendation of visual size.

925. We are used to making very rapid judgments as soon as we receive sight-sensations. We take these sensations as signs, but because they allow us to discern almost automatically the size of bodies, we also seem to perceive size itself through sight.
This false judgment is made by practically everyone, and it would not be out of place to call it a common-sense error [App., no. 36].

Errors of this kind lead to research which becomes entirely superfluous once the error has been dissipated. Let me give an example of such a pointless inquiry. With my eyes open, I can behold immense vistas. Amongst many other things making up the panorama, I catch sight of another person, dwarfed in comparison with the rest of the scene. His two eyes are tinier members of a tiny body. In each of them I notice a little black hole behind which is stretched a small, delicate and extremely sensitive background, called a retina, where light carries out its marvellous task of stimulation. On this very restricted backdrop which forms the final clothing of his eye, the other person sees me and everything else, just as I, in a similar, small, nerve-sensitive space, see him and everything else - earth, sky and immense universe. Nevertheless my eye, which sees the other person's eye, or itself in a mirror, tells me that the screen receiving the colours of so many things is no broader than a tiny line, although the things depicted in it appear immensely greater than it. How can it receive such vision? Does it deceive me by showing me objects of an immense size, when the impression it receives is so small?

The difficulty vanishes totally if we keep in mind that, as we have shown above, the eye perceives neither size nor distance, but only their signs from which the mind with a rapid judgment passes to conceive distance, while the animal acts with the shrewdness of habitual instinct as though it had conceived distance.

Signs do not have to be of the same nature and measure as the things they indicate. They enable us to know size provided we know the proportion between the size of the signs and that of the things. In the case of the eye, we know this proportion habitually because through touch we grasp the real size of things and form a habit of comparing them with the apparent size administered by the eye.

926. Another possible difficulty merits every attention. The eye is also an organ of touch, and light really touches it. Why can we not apply the law of touch to the eye? This law states that when we size up a body with our hand, which is a very suitable instrument of touch-sensations, we measure the body with the hand itself, using it as a basic unit superimposed upon the body to make the comparison. In this superimposed touch we have distinguished the sensation in the hand from the perception of the external body, and have already seen that the extension of the sensation in the hand is the measure of the extension of the body that has contact with the hand. Hence the subjective sensation of our own body is the measure of extrasubjective perception, that is, of the external agent compared in such an operation with our body. We apply this law to our eye touched by particles of light. In this case, our eye will have: 1. a subjective sensation of different parts of the retina as touched by light rays of varying breadth; 2. an extrasubjective perception of the particles of light. It will measure what acts upon it with the extension of subjective sensation, that is, the thinnest rays of light, or at least the extension of the bundles of rays that work like artists' brushes upon the screen of the eye. If we now confine our attention to the sensation of sight considered as touch, we cannot avoid noting the smallness of the depicted images and realising that they are smaller than the small aperture of the eye which is, as it were, the general scene or picture whose various parts are obviously smaller than the whole. Noting, as we must, the smallness of the images received in the eye, we must also feel the proportion they have with the eye itself. It is true that these tiny images can be signs of the true, tactile size of things, after we have learned to use our touch, just as the marks on a map are signs of the size of a territory when we know the scale of the map, but this does not weaken the validity of our first knowledge, through which we compared the little images in the eye with the eye itself and, like every other object of touch, measured them with the eye according to their own, real size. Nevertheless, we have no inkling of this in our experience.

927. The difficulty may be solved as follows. The marks in the eye should not be called images until we have noted, through touch, that the colours impressed upon the eye are signs of external bodies. Only touch can tell us this. Because the colours tinting the eye form only light-marks which do not of themselves signify or represent anything, they are neither images nor signs for us prior to the use of touch. But the simultaneous use of touch and sight enables us to discover the constant relationship between the size furnished by touch and that provided by the marks in the eye. Because these marks vary as bodies vary to the touch, they become signs for us, and appear true images of bodies.(202)

Although the eye, of itself, perceives only sensations or, as I have called them, certain colour-marks felt only in the retina, the use of touch allows these marks or sensations to function as signs of distant things and to acquire a new state or, better, relationship with us through which we consider them totally different from what they were previously. In fact they seem to take on another nature.

The marks or sensations on the retina, therefore, and the visual images are the same thing, as far as their own being is concerned, but two things as seen by us. In other words, when we consider the sensation as a mark felt in the eye, and as an image of something external, our attention is brought to bear on two entirely opposite terms: first, upon the mark we feel, that is, the sensation in the retina; second, upon the mark as image, when we move on directly to the thing represented and consider it as the only term of attention, without resting in the sign. Thus when a person sees a portrait of a friend, he thinks immediately of his friend without stopping to examine the picture in its own being. He ignores the canvas, types of paint, and other elements that compose it. This is possible because the mark felt in the eye is changed into an image through the intervention of touch and, as an image, immediately stimulates our attention to move well away from the portrait in its search for the object of which the mark is an image. But we cannot understand this most important fact without practical conviction of the supremely important distinction between sensation and advertence to sensation upon which we could say the whole of philosophical knowledge rests.(203)

928. The law governing advertence is as follows: 'That which we advert to is the term of our intellective attention.' Advertence of something arises in us when our attention moves towards and terminates in the thing in such a way that it becomes the final object of our attention. The intermediate links through which our attention and thought pass without making the links their term, are perceived fleetingly, but not adverted to. If we want to advert to them, we have to turn back and pass rapidly over the road we have taken so that the links we have previously ignored may become terms of our attention. We advert, therefore, to that which involves and terminates our act of attention; the many other things we feel and perceive remain outside our attention and inadverted.

In our present case, when the sensations experienced in the retina of the eye have acquired the quality and state of images, they cannot of themselves be terms of our attention because, as we have said above, images of their nature draw us outside themselves by becoming guides directing our attention to what they represent. An image provides a special relationship between two things, one of which serves as a scale or means for directing our thought to the other; an image, as such, moves our attention from the nature of the thing acting as image towards the object represented, which then becomes the term of attention. The sensation on our eye, once it has become a sign and quasi-image of external things, no longer holds our attention and advertence, but directs it to another term. Thus the sensation itself remains unobserved and inadverted.

929. Another consideration may be added: 'Our advertence is attracted more easily by distinct than by confused perceptions.' If we now ask what makes sense perception distinct or confused, we find three obvious reasons for its heightened clarity. Our sense perception is more distinct when bodies perceived by sense are: 1. fewer in number; 2. of sufficient size to be grasped in their entirety; 3. more stable in the forms they present to sense. But particles of light are innumerable, incalculably small, perpetually mobile, and as such capable of providing only a vivid, but altogether confused perception as they simultaneously strike the retina. Moreover, when we perceive a body in a confused manner, we seem scarcely to perceive it at all, and often say, for example, that we perceive nothing if all that we see are spaces of air illuminated by uniform light.

On the other hand, our touch-perception is by nature extremely distinct, a characteristic it shares with the vivid signs of perception furnished by the eye. These signs are quite different amongst themselves, and possess extraordinary definition in their minuteness. Consequently, while we advert scarcely, if at all,(204) to the immediate perception of particles of light and their variety in the sensations on our eye, we pay great attention to observing the bodies furnished by touch in so far as the sight-sensations signify them to us. Observing bodies in this way is immensely useful in life's daily contingencies and far removed from pointless consideration of light-marks in our eyes.

930. If objects delineated by light in the pupil are at different distances, they do not maintain their proportional size; more distant objects send a smaller image to the eye, and closer objects a larger image. This is due to the converging rays of
light; the more distant their point of departure, the more acute the angle at which they strike the eye and arouse a sensation. The result is a smaller vestige of the object on the eye than there should be. This kind of delusion must not, however, be attributed to the sensation which, as such, tells us nothing of the object. It is the judgment made by our mind that deceives us as we infer the size of exterior bodies from the sensation of light taken as a sign.

931. But this error also is soon corrected. The images coming to us from various distances follow another kind of proportion which serves to distinguish the distances themselves. Apparent size now becomes a sure sign and measure of the distances of bodies in so far as the image in the eye increases in size as the distance diminishes, and vice-versa. Apparent sizes and their distances bear a constant inverse proportion to one another. The constancy of this proportion is the foundation of the art of perspective.

Spontaneous movement and touch indicate true distances. Habitual observation enables us to know the relationship between the apparent size of bodies, and their distance measured by touch and movement. We then learn to pass with great speed from one to the other, and to note immediately, from the apparent size, the distances of bodies from one another, at least approximately.
If we stand at the end of a long drive of trees, we see an apparent decrease in the size of the trees on both sides. It is this which makes us aware of the ever-greater distance of the trees from one another, and finally of the distance between the last and the first trees.

Once I have become used to relating the height of the trees to their distance I no longer err. Decreasing size becomes for me the effect of distance and nothing else. I amend the disproportion of apparent height, and by mentally positioning the trees at the same distance, I know that they are of the same height (granted they are in fact equal).

932. Although light imprints bodies on our eyes upside down, we see them right way up because this reversal of the seen bodies is not and cannot be in contradiction with the various parts of the images themselves, nor with touch-perceptions. It contradicts only the fundamental feeling by which we feel the eye, and the modification of the fundamental feeling by which we feel our eye subjectively.

933. First of all, I note that if I perceive a bodily image upside down, the different parts of the image do not contradict the perception. The eye, by fixing its attention only on the image, cannot perceive that it is upside down.
In fact, when I turn a vase, for example, upside down, I notice its new position only through the relationship it has with the surrounding bodies which remain right way up. But let us assume that all the surrounding bodies, and we ourselves, are turned upside down in the whole image without any relative change of parts. In this case, it would be impossible for us to become aware of the new position of the vase and ourselves. No other body would remain to serve as a sign with which we could compare the change in our body. As we have seen, movement cannot be felt of itself, but only through the relationship between bodies which have been moved and perceived by us. The rotation of the earth, inverting us each day, proves the point. We have to discover this inversion through reason rather than through feeling because of the fixed position of our bodies relative to other things. The same is true about our eye. Whatever position images take in our eye, whether they are the right way up or upside down, it could never be recognised by the sense of sight alone. The images revolve together and retain their natural proportions while we ourselves, as seen, revolve with everything else. In our eye the whole world revolves, and because there is no change or contradiction between the different parts of the visual image, it is impossible to notice the inversion of particular bodies through the upside down position of their images; if the eye changes the images, everything changes together. It is like our incapacity for feeling or noticing the inversion resulting from the daily rotation of the world.

934. All the eye can do is to notice things upright, that is, in their true, natural positions relative to one another; even touch itself cannot give us any indication of the eye's upside down view of things. The position of the images on the eye, whatever it may be, cannot be in contradiction with the position of bodies felt by touch. The eye sees the relative position of bodies as it is; touch also senses the same relative position, and nothing else. For example, what is positioned above my head (this relationship establishes the position of things) is there whether I perceive it by sight or by touch. This is true whether I am standing upright, lying down, or standing on my head: the things around me remain in the same position relative to my eyes and hands. There can be no contradiction, therefore, between the position indicated by sight and by touch whatever direction may be proffered by the images traced on the sensitive 'screen' of the eye.

935. This is not the case relative to the fundamental feeling and the acquired sensation which makes us perceive the sensitive 'screen' of the eye. Here the images do contradict the position of bodies as it is given by touch. Let us suppose that an image is felt adhering to our eye so that we have an image-perception joined to the sensation of the whole eye and superimposed upon the retina. This is what takes place in touchsensation, which is always twofold because it is superimposed upon the felt surface of the hand that touches the surface of the exterior body so that one measures the other. In our supposition, I would feel the image upside down in my eye, which simply means that its position is opposite to the position of my eye. If I were now to have in my eye the image of another eye, the latter would be upside down with the eyebrows underneath, relative to my eyebrows which hold the opposite position, that is, above. If then the tiny eye depicted in my pupil were perceived by me immediately by touch, it would be an extrasubjective perception opposed, as far as position was concerned, to the subjective perception of my eye. Why, therefore, do I not notice this contradiction between the subjective and extrasubjective parts in sight-sensation?

936. The difficulty is completely resolved by my observations on sight-sensation in the preceding Article. I observed that when the eye is considered as touch, that is, as a sense that perceives colours immediately, we can no longer rightly speak of it as perceiving images but only colour-marks. Now as long as we consider colours perceived by the eye in themselves, without reference to their nature as signs, their position upon our eye means nothing to us. Consequently, reflecting on them relative to the position of the eye itself must be extremely difficult, if not impossible.

Moreover, when the colour-marks have changed into images, we no longer give them any attention, as we said. We use our eyes continually for the sole purpose of knowing exterior bodies, not for knowing what happens in our eyes. As a result of this continual attention to external objects that we see, we are incapable of concentrating our attention on the eye and on the change that it undergoes.

937. In the second place, although the extrasubjective lightsensation is strong, it is still not easy for us to measure the size of the very restricted subjective sensation. Furthermore, it is impossible to advert to its position relative to our sentient eye. In fact, to know and advert to one position of the image relative to my eye rather than another, I must: 1. note the position of the colour-mark; 2. note and advert to the position of my eye; 3. compare these positions; 4. note which parts of the mark represent to me the extremities of the external thing; 5. note and advert that the part of the mark representing the top extremity of the external thing corresponds to the low part of the eye, and vice-versa. All these operations are extremely difficult, and probably impossible. To avoid an endless task, it would be well for me to comment only on the difficulty of the third step, where the position of the colour-mark is compared with the position of my eye. I feel this position with my fundamental feeling alone, and feel the position of the mark with the acquired sensation. We have already seen how difficult it is to advert to the fundamental feeling, and this difficulty would be compounded if we had to advert to the relative position of the parts felt in the fundamental feeling with the clarity, distinction(205) and firmness needed to compare it with the position of adventitious sensations or of the colour-marks we are discussing.

938. Everything I have said explains why I cannot agree with those ideologists who say we first see things upside down and then turn them the right way up. On the contrary, we always see things the right way up and cannot see them any other way. As far as I can understand, it is impossible, even with the most acute advertence, to succeed in noting through sight alone the following extraordinary fact: 'When we take the shape of the sensation as a sign of the external body, the lowest point of the sensation in our eye indicates the highest point of the external thing, and vice-versa' [App., no. 37].

939. Touch, united with spontaneous movement, perceives extension immediately (cf. 837-875).
Hence it is this sense that perceives the limits of extension, size, shape.(206)
It follows that the shape of things perceived by touch and movement is the criterion against which to compare the shape ministered by sight.

940. Light enables us to perceive distant bodies because they refract and reflect it to us in such a way that its modifications are proportioned to the size, shape, distance and other qualities or conditions of the bodies themselves.
But rays of light can be deviated or altered as they pass from bodies to ourselves if they meet something on the way, or can become accidentally united. In these cases, the impression they give does not correspond to the shape we already know and use as a faithful guide to judge the bodies, which we now judge falsely because the light does not faithfully present them. Hence such optical illusions as branches bent in water, or pebbles appearing as rocks in very cold climates where the condensed air acts as a magnifying glass, and other mistakes discovered and corrected by touch.

Notes

(201) The philosopher who declared our body to be the measure of all things would have made a truly remarkable
affirmation if he had confined his assertion to the size of spaces and bodies.

(202) I say appear, because their only likeness with external things lies in their extrasubjective element.

(203) Depending upon circumstances, I call advertence: observation, attention, consideration and awareness. All these words express an intellective act, fixed upon a sensation, which forms an idea and adverts to the sensation. Galluppi states correctly that ideas are formed by meditation on sensations, but does not tell us the nature of this meditation, reflection, or action of the understanding. It can only be the application of a universal idea to sensations (cf. 482-489); without this, meditation has no meaning and reflection is inexplicable.

(204) I say 'scarcely, if at all', rather than 'not at all', because everyone can notice some sensation in the eyes. We feel light falling upon our eyes, and find quite a difference in our pupils when we close our eyes. But, as I said, we do not advert to what takes place in our eyes when we have so many beautiful things to look at.

(205) I also think it altogether impossible to advert distinctly in the fundamental feeling to the relative position of its parts without the help of acquired sensations. Can one say, in fact, that the fundamental feeling has clearly distinguishable parts?

(206) Space does not change shape for the same reason that it does not change size. Two different shapes are only two independent pieces of space. One space, therefore, can never be transformed into another. A shape in space cannot rightly be said to change into another. If succeeded by another, it is not what it was. The second is an altogether new shape, not the first transformed.