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.