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’?il, 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 coul............