It is also well to observe that we do not refer the motion perceived by touch to the objects around us until after we have acquired this habit by means of repeated comparisons. When for the first time the hand is passed over an object, we are unable to tell whether the hand moves over the body, or the body under the hand.
The reason of this is that the sensation of motion is essentially a successive sensation, and this succession exists equally whether the hand moves or the object. Let us suppose the hand to pass along an object of a varied surface, we shall experience the variety of sensations corresponding to the surface; suppose now that the hand remains motionless, and the object passes under it with the same velocity, pressure, and friction, the sensations will be the same as before. Every one must have observed that when leaning on a slippery object, it is often difficult to tell whether it is the object which moves, or ourselves. Therefore touch also confirms what we have advanced, that the distinction between the motion of the member and that of the object does not arise from simple sensation.
88. In this respect, therefore, touch does not help sight; let us see if sight alone can enable us to distinguish between the motion of the eye and that of the object. We have already observed that a single sensation with respect to one object only is insufficient, but there is no difficulty in proving that this result may be obtained by the comparison of different sensations.
Let us suppose the eye at a point A, looking at an object B; the object will appear at the extremity of the range of the sight as if projected on a plane. To be more definite we will imagine the object B to be a column in the middle of a large hall, and the point A a corner of the same hall. The column will appear to the eye to be a part of the opposite wall. If the eye changes its position, the column will appear in another part of the wall; so that if the eye should pass around the column, it would appear successively on every part of the wall. The same succession of phenomena would be observed if the eye should remain fixed and the column should move around it; for it is evident that if the observer is placed in the centre of the room, and the column moves around him, the column will appear on all the parts of the opposite wall. From this we infer that only one sensation of sight, with only one object, is not enough to determine whether the eye moves or the object.
But if instead of one object we suppose several moving simultaneously, it is easy to see how the distinction of motions arises. Let us suppose that at the same time that the eye sees the column, it also sees other objects, such as chandeliers, statues, or other columns, placed between the eye and the opposite wall. If the eye moves every time the column changes its position on the opposite wall, the other columns, the chandeliers, statues, everything in the room seems to change its position; whereas, if the column moves and the eye remains fixed, the column alone changes its position, while everything else remains motionless. Therefore sight alone gives us two distinct orders of phenomena of motion:
I. The first, in which all the objects change their position.
II. The second, in which one object only changes its position.
These two orders of phenomena cannot remain unperceived; for by the help of reflection excited and enlightened by the repetition of the phenomena, we must come to the conclusion that when there is an entire and constant change of all the objects, it is not they that move, but the eye; and that when only one or a part of the objects change their position, the rest remaining fixed, it is not the eye that moves, but the objects which change their position. When everything around us changes we infer that it is the eye that moves; when one or two change their position we conclude that they move and not the eye. This is not merely a supposition, it is the reality. The ideas derived from touch are essentially limited, and it is therefore impossible that they should proceed from distant objects which cannot be touched.
89. I believe I have demonstrated that the pretended superiority of touch is without foundation, and that the opinion which makes this the basis of our knowledge of external objects, the touch-stone of the certainty of the sensations transmitted by the other senses, is an error. Without it we can acquire the certainty of the existence of bodies; without it we can form the idea of surfaces and solids; without it we discover motion, and distinguish the motion of the object from that of the organ which receives the impression. The theory of sensation here explained, and the results which are deduced from the relations of the dependence or independence of the phenomena among themselves, and with our will, may all be applied to the sight as well as to the touch.
90. Summing up all we have said, we have the following results: