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.