41. But because this is contrary to appearance, it may seem not to merit belief unless it be proved; and since it can be proved only by such things as man can perceive from his bodily senses, it will be proved by these. Man has five external senses which are called touch, taste, smell, hearing and sight. The subject of touch is the skin by which man is enveloped. The very substance and form of the skin cause it to feel the things applied to it. The sense of touch is not in the things applied, but in the substance and form of the skin which are the subject. That sense is only an affecting of it by the things applied. It is the same with taste. This sense is only an affecting of the substance and form which belong to the tongue; the tongue is the subject. It is the same with smell. It is well known that odour affects the nostrils, and that it is in the nostrils, and smell is an affecting of them by the odoriferous things touching them. It is the same with hearing. It appears as if the hearing were in the place where the sound originates, but the hearing is in the ear, and is an affecting of its substance and form. That the hearing is at a distance from the ear is an appearance. It is the same with sight. When a man sees objects at a distance, the seeing appears to be there, but yet the seeing is in the eye which is the subject and is likewise an affecting of it. Distance is solely from the judgment inferring about space from intermediate things or from the diminution and the consequent indistinctness of the object, an image of which is produced interiorly in the eye according to the angle of incidence. From this it is evident that sight does not go out from the eye to the object, but that the image of the object enters the eye and affects its substance and form. Thus it is just the same with sight as it is with hearing. Hearing does not go out from the ear to catch the sound, but the sound enters the ear and affects it. From these considerations it can be established that the affecting of substance and form which causes the sense is not a something separate from the subject, but only causes a change in it, the subject remaining the subject as before and afterwards. Hence it follows that sight, hearing, smell, taste and touch are not a something volatile flowing out of their organs, but that they are the organs themselves, considered in their substance and form, and that when the organs are affected, sensation results.