Philosophical Dictionary, by Voltaire


A man who knows how to reckon the paces from one end of his house to the other might imagine that nature had all at once taught him this distance and that he has only need of a coup d’œil, as in the case of colors. He is deceived; the different distances of objects can be known only by experience, comparison, and habit. It is that which makes a sailor, on seeing a vessel afar off, able to say without hesitation what distance his own vessel is from it, of which distance a passenger would only form a very confused idea.

Distance is only the line from a given object to ourselves. This line terminates at a point; and whether the object be a thousand leagues from us or only a foot, this point is always the same to our eyes.

We have then no means of directly perceiving distances, as we have of ascertaining by the touch whether a body is hard or soft; by the taste, if it is bitter or sweet; or by the ear, whether of two sounds the one is grave and the other lively. For if I duly notice, the parts of a body which give way to my fingers are the immediate cause of my sensation of softness, and the vibrations of the air, excited by the sonorous body, are the immediate cause of my sensation of sound. But as I cannot have an immediate idea of distance I must find it out by means of an intermediate idea, but it is necessary that this intermediate idea be clearly understood, for it is only by the medium of things known that we can acquire a notion of things unknown.

I am told that such a house is distant a mile from such a river, but if I do not know where this river is I certainly do not know where the house is situated. A body yields easily to the impression of my hand: I conclude immediately that it is soft. Another resists, I feel at once its hardness. I ought therefore to feel the angles formed in my eye in order to determine the distance of objects. But most men do not even know that these angles exist; it is evident, therefore, that they cannot be the immediate cause of our ascertaining distances.

He who, for the first time in his life, hears the noise of a cannon or the sound of a concert, cannot judge whether the cannon be fired or the concert be performed at the distance of a league or of twenty paces. He has only the experience which accustoms him to judge of the distance between himself and the place whence the noise proceeds. The vibrations, the undulations of the air carry a sound to his ears, or rather to his sensorium, but this noise no more carries to his sensorium the place whence it proceeds than it teaches him the form of the cannon or of the musical instruments. It is the same thing precisely with regard to the rays of light which proceed from an object, but which do not at all inform us of its situation.

Neither do they inform us more immediately of magnitude or form. I see from afar a little round tower. I approach, perceive, and touch a great quadrangular building. Certainly, this which I now see and touch cannot be that which I saw before. The little round tower which was before my eyes cannot be this large, square building. One thing in relation to us is the measurable and tangible object; another, the visible object. I hear from my chamber the noise of a carriage, I open my window and see it. I descend and enter it. Yet this carriage that I have heard, this carriage that I have seen, and this carriage which I have touched are three objects absolutely distinct to three of my senses, which have no immediate relation to one another.

Further; it is demonstrated that there is formed in my eye an angle a degree larger when a thing is near, when I see a man four feet from me than when I see the same man at a distance of eight feet. However, I always see this man of the same size. How does my mind thus contradict the mechanism of my organs? The object is really a degree smaller to my eyes, and yet I see it the same. It is in vain that we attempt to explain this mystery by the route which the rays follow or by the form taken by the crystalline humor of the eye. Whatever may be supposed to the contrary, the angle at which I see a man at four feet from me is always nearly double the angle at which I see him at eight feet. Neither geometry nor physics will explain this difficulty.

These geometrical lines and angles are not really more the cause of our seeing objects in their proper places than that we see them of a certain size and at a certain distance. The mind does not consider that if this part were to be painted at the bottom of the eye it could collect nothing from lines that it saw not. The eye looks down only to see that which is near the ground, and is uplifted to see that which is above the earth. All this might be explained and placed beyond dispute by any person born blind, to whom the sense of sight was afterwards attained. For if this blind man, the moment that he opens his eyes, can correctly judge of distances, dimensions, and situations, it would be true that the optical angles suddenly formed in his retina were the immediate cause of his decisions. Doctor Berkeley asserts, after Locke — going even further than Locke — that neither situation, magnitude, distance, nor figure would be discerned by a blind man thus suddenly gifted with sight.

In fact, a man born blind was found in 1729, by whom this question was indubitably decided. The famous Cheselden, one of those celebrated surgeons who join manual skill to the most enlightened minds, imagined that he could give sight to this blind man by couching, and proposed the operation. The patient was with great difficulty brought to consent to it. He did not conceive that the sense of sight could much augment his pleasures, except that he desired to be able to read and to write, he cared indeed little about seeing. He proved by this indifference that it is impossible to be rendered unhappy by the privation of pleasures of which we have never formed an idea — a very important truth. However this may be, the operation was performed, and succeeded. This young man at fourteen years of age saw the light for the first time, and his experience confirmed all that Locke and Berkeley had so ably foreseen. For a long time he distinguished neither dimensions, distance, nor form. An object about the size of an inch, which was placed before his eyes, and which concealed a house from him, appeared as large as the house itself. All that he saw seemed to touch his eyes, and to touch them as objects of feeling touch the skin. He could not at first distinguish that which, by the aid of his hands, he had thought round from that which he had supposed square, nor could he discern with his eyes if that which his hands had felt to be tall and short were so in reality. He was so far from knowing anything about magnitude that after having at last conceived by his sight that his house was larger than his chamber, he could not conceive how sight could give him this idea. It was not until after two months’ experience he could discover that pictures represented existing bodies, and when, after this long development of his new sense in him, he perceived that bodies, and not surfaces only, were painted in the pictures, he took them in his hands and was astonished at not finding those solid bodies of which he had begun to perceive the representation, and demanded which was the deceived, the sense of feeling or that of sight.

Thus was it irrevocably decided that the manner in which we see things follows not immediately from the angles formed in the eye. These mathematical angles were in the eyes of this man the same as in our own and were of no use to him without the help of experience and of his other senses.

The adventure of the man born blind was known in France towards the year 1735. The author of the “Elements of Newton,” who had seen a great deal of Cheselden, made mention of this important discovery, but did not take much notice of it. And even when the same operation of the cataract was performed at Paris on a young man who was said to have been deprived of sight from his cradle, the operators neglected to attend to the daily development of the sense of sight in him and to the progress of nature. The fruit of this operation was therefore lost to philosophy.

How do we represent to ourselves dimensions and distances? In the same manner that we imagine the passions of men by the colors with which they vary their countenances, and by the alteration which they make in their features. There is no person who cannot read joy or grief on the countenance of another. It is the language that nature addresses to all eyes, but experience only teaches this language. Experience alone teaches us that, when an object is too far, we see it confusedly and weakly, and thence we form ideas, which always afterwards accompany the sensation of sight. Thus every man who at ten paces sees his horse five feet high, if, some minutes after, he sees this horse of the size of a sheep, by an involuntary judgment immediately concludes that the horse is much farther from him.

It is very true that when I see my horse of the size of a sheep a much smaller picture is formed in my eye — a more acute angle; but it is a fact which accompanies, not causes, my opinion. In like manner, it makes a different impression on my brain, when I see a man blush from shame and from anger; but these different impressions would tell me nothing of what was passing in this man’s mind, without experience, whose voice alone is attended to.

So far from the angle being the immediate cause of my thinking that a horse is far off when I see it very small, it happens that I see my horse equally large at ten, twenty, thirty, or forty paces, though the angle at ten paces may be double, treble, or quadruple. I see at a distance, through a small hole, a man posted on the top of a house; the remoteness and fewness of the rays at first prevent me from distinguishing that it is a man; the object appears to me very small. I think I see a statue two feet high at most; the object moves; I then judge that it is a man; and from that instant the man appears to me of his ordinary size. Whence come these two judgments so different? When I believed that I saw a statue, I imagined it to be two feet high, because I saw it at such an angle; experience had not led my mind to falsify the traits imprinted on my retina; but as soon as I judged that it was a man, the association established in my mind by experience between a man and his known height of five or six feet, involuntarily obliged me to imagine that I saw one of a certain height; or, in fact, that I saw the height itself.

It must therefore be absolutely concluded, that distance, dimension, and situation are not, properly speaking, visible things; that is to say, the proper and immediate objects of sight. The proper and immediate object of sight is nothing but colored light; all the rest we only discover by long acquaintance and experience. We learn to see precisely as we learn to speak and to read. The difference is, that the art of seeing is more easy, and that nature is equally mistress of all.

The sudden and almost uniform judgments which, at a certain age, our minds form of distance, dimension, and situation, make us think that we have only to open our eyes to see in the manner in which we do see. We are deceived; it requires the help of the other senses. If men had only the sense of sight, they would have no means of knowing extent in length, breadth, and depth, and a pure spirit perhaps would not know it, unless God revealed it to him. It is very difficult, in our understanding, to separate the extent of an object from its color. We never see anything but what is extended, and from that we are led to believe that we really see the extent. We can scarcely distinguish in our minds the yellow that we see in a louis d’or from the louis d’or in which we see the yellow. In the same manner, as when we hear the word “louis d’or” pronounced, we cannot help attaching the idea of the money to the word which we hear spoken.

If all men spoke the same language, we should be always ready to believe in a necessary connection between words and ideas. But all men in fact do possess the same language of imagination. Nature says to them all: When you have seen colors for a certain time, imagination will represent the bodies to which these colors appear attached to all alike. This prompt and summary judgment once attained will be of use to you during your life; for if to estimate the distances, magnitudes, and situations of all that surrounds you, it were necessary to examine the visual angles and rays, you would be dead before you had ascertained whether the things of which you have need were ten paces from you or a hundred thousand leagues, and whether they were of a size of a worm or of a mountain. It would be better to be born blind.

We are then, perhaps, very wrong, when we say that our senses deceive us. Every one of our senses performs the function for which it was destined by nature. They mutually aid one another to convey to our minds, through the medium of experience, the measure of knowledge that our being allows. We ask from our senses what they are not made to give us. We would have our eyes acquaint us with solidity, dimension, distance, etc.; but it is necessary for the touch to agree for that purpose with the sight, and that experience should second both. If Father Malebranche had looked at this side of nature, he would perhaps have attributed fewer errors to our senses, which are the only sources of all our ideas.

We should not, however, extend this species of metaphysics to every case before us. We should only call it to our aid when the mathematics are insufficient.

Last updated Sunday, March 27, 2016 at 12:01