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.