CHAPTER 12

Origin of the idea of bodies
through the extrasubjective perception of sight

906. Let us imagine a human being standing still with eyes open. Vision in this person is limited to a variously-coloured surface adhering to his eyes, without background or perspective.

907. Because body is an agent producing feeling with an extension mode, feelings located at points in space are corporeal actions. But our coloured surface is a feeling extending over a surface. It is therefore corporeal.

908. All senses are touch (cf. 744-745) and as such are subject to the laws governing touch; they differ amongst themselves only through their accidental phenomena. Our study of these phenomena showed that the sensations of our four organs possess, as a general characteristic, highly developed subjectivity with limited, confused extrasubjectivity (cf. 887-895). Such phenomena, therefore, are simply the mode of these four species of sensation; and indeed touch itself furnishes similar phenomena (ibid.), although not so distinctly. Phenomena of this kind add nothing that is capable of altering the common laws to which touch in general is subject.
In touch, however, the touching surface of the external body forms a unity with the touched surface of our body. As a result, the same surface is felt simultaneously in two ways: in our body, subjectively, and as the term of perception of the external agent, extrasubjectively (cf. 841).
It is clear, therefore, that 'the coloured surface perceived by the eye is identical with the surface of the retina touched by the light.'
We have to consider carefully the fact that the eye perceives the coloured surface in the same way as touch perceives hardness and resistance in an extended body.
In corporeal vision, therefore, we must distinguish: 1. the sensation of the retina; 2. the entire confused perception of the innumerable particles of light which fill the retina in which they are spread.

909. This extraordinary, but irrefutable truth is a corollary of the preceding affirmation.
Nevertheless, it is sometimes called in doubt as a result of inadequate observation, because of our habit of attributing to bodies perceived visually the same size that we perceive in them through touch and movement. Later I shall explain how
his habit arises and show that it depends upon the judgment we add to the sensation of sight, and not upon the sensation
tself.

910. For the time being, we first notice that, whatever the size of the agents perceived by the eye, the eye indubitably perceives them in a definite proportion relative to one another. For example, while my eye receives all the colours of the agents in its view, it can also receive those of another person's eye. But his pupil is perceived as considerably smaller than his body, which in its turn is perceived as smaller than the room in which he is standing. The reason is that his pupil occupies less of my retina than his body, and his body less than the light-filled room.
The eye, therefore, perceives the relative sizes of bodies that are equidistant from it, although it does not perceive their absolute size.
People born blind who later gain sight can confirm these observations. In the first moments of their use of sight, they experience a sensation adhering to the retina of their eye, but no distance or real distinction of external bodies. What they perceive is a painted canvas, that is, the surface of their retina covered with varying light (cf. 811).

911. We have already seen that the eye perceives movement. But any change whatsoever, taking place in the coloured surface we perceive, is reduced to change in the surface itself. The succession of coloured surfaces provides no idea of distance or depth; pictures succeed one another in the eye like the scenes offered by a magic lantern. By itself, therefore, the eye cannot form an idea of three-dimensional space.

912. So far we have supposed that only the eyes have been used, but not touch or movement, to discover what the eyes contain and what occurs in them. We have tried to find what term the eye can achieve when left to itself. We saw that, without movement or touch, a person would perceive a coloured surface adhering to his eye; it would be no larger than the retina affected by light, and would stimulate sensation (cf. 909). We also observed that in this tiny surface colours are spread out and divided in a certain order, not haphazardly; the same can be said about the movements taking place in them. The colours are in certain proportions, corresponding precisely to the proportions in the sizes of the external things furnished by touch (ibid.).
The constancy of these proportions and the order maintained in the movements of the perceived colours is of great benefit in permitting the colours to act as signs by which we may learn the true sizes(194) of things, and the distances and quantities of movement in our own body.

913. Let us examine what takes place relative first to the size of external things, and then to distances and quantities of movement. External things transmit light to our eye from every point of their surface. Larger things transmit a greater number of rays which, when the things are equidistant from the pupil, cover a greater area of the pupil. Things seen at the same distance, therefore, are indicated and depicted by sizes proportionate to that which they possess in themselves.(195) The pattern of things imprinted by the light on our retina, resembles a map; its scale, although less than the reality, perfectly preserves the proportions between the parts found in reality. In the same way, external bodies are depicted in a smaller scale on our retina without changing the proportions in any way. The eye and light cooperate so well in drawing visible things on a lesser scale but in constantly equal proportion that the instruments used to reduce a larger to a smaller scale are only an imitation of what is done more perfectly by nature.

914. This example of the map is very helpful for our present purposes. When we look at a map, we pay immediate attention not to the colours or other qualities reproduced there, but to the scale of what we see, which indicates the real size of the area depicted. In the same way, it is not the quality of the colours that provides true, immediate knowledge of what we see in the variety of colours perceived by the eye in any sensation; colour, as such, is the subjective part of sensation,(196) as we have seen. The size and proportion of the different coloured spaces is the extrasubjective part, which indicates the size of exterior things. It offers a true likeness of them: a small triangle or square truly resembles a large triangle or square; the proportion between a city and a house is equal to that between the two symbols, which stand for the relationship between the city and the house.(197) In the same way, the eye indicates the size of things through a likeness of the sensation to them, and not through their other properties.

Now, if we wish to see how we come to know the size of things from the colour sensations experienced in our eyes, we have to begin by employing our touch. Here we suppose that, with this sense and with movement, we have already perceived external bodies along with their absolute extensions and their proportions. Using touch and sight simultaneously, anyone can notice an extraordinary relationship between the parts of bodies perceived by touch, and the colours perceived by sight. My hand, held out to touch a body, removes a colour from my sight; every point that it touches is a spot hidden from me because my hand covers it. By repeating these experiences, I finally learn that the sensations of touch and sight are stably related to one another, and realise that a touch-sensation outside myself corresponds to every coloured point in my eye. If one of the lightmarks affecting my retina is larger, my hand can move further with its touch to cover it. Touches like this are continuing perceptions of external bodies, and serve, as we have seen, as a measure of their size. Because every coloured point of the eye corresponds to the touch-perception of a body, and every more or less large light-mark corresponds constantly and proportionately with the touch-perception of different sized bodies, it must and does occur that the marks on the eye from different rays of light are sure indications and signs of external bodies and their size, which only touch perceives immediately. We thus form a habit of passing with extreme rapidity of thought from sight-sensations to persuasion about external, touchable bodies. This habit, which never ceases in us, is strengthened and developed to such an extent that we confuse and exchange the signs with what they signify and say as soon as we perceive a light-mark with the eye: 'I see a body, a touchable object,' instead of: 'I perceive a light-mark, which assures me of a touchable body outside me.'(198)

When we look at a map, we know the size of the places indicated provided we have a clear idea of the scale on which they are depicted. However, it is much easier to measure the size of things on the 'map' supplied by the eye than to recognise almost intuitively from a topographical map the size of the area under examination. The reason is clear: our visual map is always before us and, with the help of touch to correct and test the sizes it shows, is being continually applied to various situations.

915. There is another difference between seeing a country on a map and perceiving external bodies through the perception of the retina invaded by varying colours of light-marks from the light refracted by bodies and reflected to the pupil. The map is totally separate from the country shown on it, without any lines, so to speak, tying it to the country. On the other hand, the picture in the eye has an admirable, physical connection with bodies perceived by touch: rays of light emanating from bodies join them to the impressions experienced by the eye. It is not a question, of course, of the eye being drawn outside itself by these rays of light passing from the bodies to itself, nor of its perceiving anything other than the extremities of the rays. The extremities are changed with lightning speed and accuracy by every movement in the bodies that communicate them to the eye, especially by hands touching the bodies. Because experience teaches children that they have a light-sensation for every point touched by their hands, the points of light felt by their eyes are commensurate with those touched by their hands. They are thus led to identify visual measure with that of touch by superimposing one on the other, point by point, as it were, line by line, surface by surface. Experiences of this kind, provided by nature herself, allow us to find without difficulty in the coloured light-marks of the eye, the measure itself of bodies as given through touch-perception.

916. Yet another difference between a map and its countries, and between tactile bodies and the retina speckled with colour, will help to explain the fact under consideration. The countries as such and the map are both terms of sight, one larger than the other. An external body and colours, on the other hand, are both terms of touch, but of touch in two different parts of our body. One of these parts, the pupil, is extremely delicate and far more complex than the part connected with ordinary touch. This difference has given to sight its own particular name, separating it from touch. Now as long as we are dealing with two terms of sight such as two triangles, one much larger than the other, their likeness enables one to be a sign of the other. Nevertheless, their unequal sizes cannot be easily disregarded; there is an obvious difference between the triangles. This is not the case with the coloured surfaces perceived by the eye and the surfaces perceived by touch, both of which manifest extremely different sensible qualities. Their likeness in form and their diversity in size cannot be easily noticed without, so to speak, superimposing one on top of the other. But nature prevents this and provides instead a kind of special, deceptive superimposition so that, when our hands touch visible bodies, we seem to superimpose the apex of pyramids of light in our eye on the objects we touch. In fact, however, we superimpose the base of the pyramid which we do not perceive. What happens is that we mentally connect the apex we perceive with the base we do not perceive.
This explains why it is more difficult for us to recognise the difference in size between what is seen and touched than to believe in their equality.

917. Let us now imagine we are in motion with our eyes open.(199) The changes caused in our sight sensations by this movement consist in a constant change of colour, and change from obscurity to clarity and vice-versa. If you look from a distance at the colour and form of a great building, it will perhaps appear as an indistinguishable whitish point against the blue of a high mountain behind it. As you move towards it, the white point grows bigger and gradually takes shape as its outline becomes sharper. As you get near it, you see it in all its size. Your movement causes the points or marks of the coloured surface (the only thing your eye sees) to expand, become distinct and take shape. But these changes are in constant relationship with the different movements you make, as we have seen.
Movement has no likeness to colour; the two are as different as taste and sound. Nevertheless, the constant relationship of colours, especially of light and shade, with movement, allows the variation of colour to present a clear sign for knowing and measuring movement itself.

918. Colours thus become a kind of language used by nature to speak to us of distance and size. This natural language is taught in the same way as the language we learn from one another.
In artificial language we use words to express ideas, although words are material sounds without any likeness to ideas, which are thoughts belonging to the spirit. Words are functional signs of our ideas. As soon as we hear them, force of habit brings to mind the ideas they represent. We form a single object of thought from ideas and words. This comes about because of the constant, analogical relationship we have created between things which differ as greatly as ideas and articulate sounds; it is this relationship which enables words to function as we have described. The same thing takes place with colours as a result of light and shade. They become quasi-words indicating the distance of things from us, and the movement carried out or required to approach things; they are analogous to what they signify.

Another likeness will help to explain more easily the perception of distance by the eye, or rather by animal perspicacity. Colours impressed upon our retina can be considered equivalent to letters of the alphabet which I write on paper but have no similarity, or even material resemblance, with the sounds called words caused by use of my speech organ as it sends out variations in air-waves. Nevertheless, despite the lack of similarity, the written curves and strokes and dots and crosses call forth words and ideas for the reader through the constant relationship, partly arbitrary and partly analogous, between the ink marks and the sounds indicating ideas. This relationship is a rule according to which thought passes with extreme rapidity from the perception of writing on paper to what the writer wished to convey.
The same is true of colours and movement. Although they have no natural resemblance, their analogous relationship enables us to use colours as signs for knowing and measuring movement, as an animal does with its natural instinct.

919. Just as we have to learn from society how to speak and write, so we have to learn from nature how to discern distance and movement with the eye.(200) After learning the art of reading distance with the eye, and the use of colours as signs of movement, we gradually perfect our habit of interpreting the signs until we think that with sight we see distance immediately and measure the movement needed to travel it. The truth is, however, that we never see anything with our eye except a surface, although the speed with which we unite the idea of extension in depth to the various colours of this surface is such that the surface finally escapes our attention. We then believe we see depth immediately, just as a reader thinks he perceives the words immediately, or a listener thinks he receives images and ideas with ears that perceive only words.

920. These three species of sensation cannot be signs as precise and general as colours enabling us to know the presence and distance of bodies, because smells, tastes and sounds do not mark off for us a corporeal surface as distinct and as continuous as that provided by the eye. Instead, they offer indistinct, changeable, perfectly homogeneous and uniform corporeal points. Moreover, because the normal objects of touch do not have the same relationship with the ears, palate and nostrils as with the eye, these sensations cannot be authenticated, as it were, by touch.

921. However, hearing does furnish a variety of sensations which, although without the intimate connection of colours to touch, are governed by fixed, simple laws which enable such sensations to be available for the formation of language. As the eye becomes a natural, although limited, language through touch (things seem to speak to us directly through ordered colours), so hearing offers a means for the discovery of a universal language.

(194) That is, those provided by touch, as we have seen, and will explain more fully in the next chapter.

(195) If this theory is clearly understood, we have an answer to Molineux's question 'whether a sphere, already distinguished from a cube by touch, can be distinguished solely by sight'. The eye itself is also touch; it perceives shape just as well as touch does by hand, although with one less dimension. Thus, in the case of the sphere and cube, one of the signs impressed by the light on the retina is circular, the other rectangular. The difference between the signs is like the difference found by touching with the hand. Hence Leibniz's affirmative answer to the question is certain.

(196) Colours also indicate the qualities of things, although as the subjective part of sensation they have no likeness to things. But on the basis of our experience, they do serve as signs. The written word, for instance, is a sign of the spoken word to which it bears no resemblance, although a portrait is the sign of the person whom it resembles. Colours thus enable us to know innumerable things - whether fruit is ripe, whether a human being is healthy or sick, what kind of mood another person is in, to mention only a few. Yet colour bears no resemblance to ripeness, health, depression or other qualities which it indicates through an association of ideas. Experience has shown us that the colour of a particular thing is joined to its qualities, so that whenever we see the colour we immediately understand the qualities. Sensation, therefore, as subjective can be a sign, but not a likeness of external things; as extrasubjective, it is a sign bearing a resemblance to things.

(197) I would like to state categorically, once and for all, that I am speaking metaphorically in referring here and elsewhere to the marks formed on the eye by colours. There is no question of impressing on the eye real marks serving as objects to be seen by others, but of subjective sensations, indicated by those marks. If I want to speak of a yellow sensation of a certain size, I speak of a yellow mark -and so on for other colours. I do not want my use of figurative language, intended to facilitate the argument, to be a cause of equivocation.

(198) Notice that we never stop to consider signs once they are well-known and their use habitual. We go directly to the things signified which we appear to see and perceive in the signs themselves. Signs seem so identified with the things they indicate that it becomes very difficult to distinguish one from the other. Hence we say, for example, that 'we have heard some truth or other from so-and-so, an expert in his field', as though we had heard the truths themselves and not simply the words alone, which bear no resemblance whatsoever to the truths we have heard. We speak of a portrait as though the person herself had been depicted, and give it her own name, because we no longer confine our attention to the portrait. We think the thing in its sign; and this occurs universally in all our operations as intelligent beings.

(199) It is the task of anthropology to explain how an animal can move in space before it has perceived space by means of its external senses.

(200) Accurate observation is needed of the time required by children to learn the connection of the size furnished by the eyes and that coming from touch and distance. It should be noted that such a connection can be obtained in two ways, instinctively and intellectually. Hence, perceiving the proportions between these sizes depends upon educating: 1. the sensitivity, which takes place in animals also; and 2. the understanding, which is proper to human beings. Sensitivity learns about the connection practically through associations of sensations, phantasms, feelings, instincts and habits, all of which in human beings are accompanied by judgments. Experiments with children should help to distinguish the progress of each of these faculties, but this is extremely difficult. Cabanis claims to have seen a deranged boy totally incapable of knowing distances by his sight alone, although his eyes were perfectly healthy (Rapports du physique et du moral de l'homme, etc., Mem. 2). If this is true, the boy must have been defective in his animal instinct as well as in intellect.