65. There are two senses which inform us with certainty of extension, sight and touch. We shall not now inquire if that be a true _sensation_ which we have of extension, or if it be an _idea_ of a very different order, resulting from the sensation. I propose hereafter to examine this point, but shall now confine myself to comparing sight with touch only as tending to give us the sensation of extension, or, if you will, to furnish us with what is necessary to form an idea of it. We cannot but see that extension lies within the domain of touch, and that, too, whether it be considered only as a surface, or also as a solid. The same faculty cannot be denied to sight, so far as surfaces are concerned; for it is impossible to see if at least a plane be not presented to the eye. A point without extension cannot be painted upon the retina, but the instant an object is painted, it has painted parts. We can by no effort of our imagination, conceive colors without extension; for what is color without a surface over which it may extend? 66. So hostile was Condillac to the sense of sight that he was unwilling to allow it even the faculty of perceiving extension in surfaces; but as he is of all philosophers the one who has most contributed to the propagation and establishment of this opinion, we will examine his doctrine and its fundamental reasons. We have only to read the chapters in which he explains it, to see that he was not himself very confident of its truth, but that he felt himself contradicted by both experience and reason. In his _Traité des Sensations_,[36] where he examines the ideas of a man limited to the sense of sight, he says that colors are distinguished by the sight because they seem to form a surface of which the eyes occupy a part, and then asks: "Will our statue, judging itself to be at one time many colors, perceive itself as a sort of colored surface?" We must bear in mind that, according to Condillac, the statue confined to one sense will believe itself the sensation, that is, it will think that it is the odor, the sound, or the taste, according as the sense of smell, hearing, or taste, is the sense in exercise; for which reason, if a surface enter into the sensation of sight, the statue ought to believe itself a colored surface. I shall not examine the correctness of these observations, but shall confine myself to the main point, which is the relation of sight to a surface. [36] P. 1. C. xi.