Wherefore the sound theory is that the configurations of things stand out in the transparent ambient--if it be effectively transparent on the shining of a luminant upon it--and hence they do not appear but in a polished body capable of receiving them, such as mirrors and the like; and so too there is in the eye a crystalline lens (or humor) into which the forms (pictures) of things are imprinted, just as their impression into mirrors; and in it, i.e., the lens or the eye, has been fitted up the seeing power; so that, if such forms are imprinted in it, it perceives them. Moreover, the objects of perception belonging in truth and deed to sight are the Colors. As for the Hearing Power: it hears only sound. And sound is a motion of air that the ear feels on two hard smooth bodies coming quickly close up one to the other, the escaping of the air from between them, its striking the ear, and its moving the air that is kept ready within the instrument (organ) of hearing. Thus, if this inside air move the instrument, and if this instrument’s motion act upon the nerve of hearing, the hearing power (sense, faculty) perceives it in the measure of the strength or weakness of that motion. Indeed hardness is a _conditio sine qua non_; for, in the case of two soft bodies, the air will not escape from them, but will dissipate itself throughout their pores. Smoothness too is just such a condition; because, in the case of rough (unsmooth) bodies, not the whole of the air will escape from between them suddenly and violently, but will be witheld (shut up) in the passages. And rapidity of contact also is a like condition; for if it come about gently and slowly, the air would not escape violently. The echo too will arise from the rebound of the air escaping from between the two encountering bodies by reason of its hitting (slapping) against another hard, flat or hollow body filled with air, because of the air that is within it hindering the penetration of the escaped air, and the latter’s striking the ear [again] after the first stroke; on the same wise as in the first instance. As to the Smelling Power; it smells odors on the sniffing in of air that has received its odor from an odoriferous body, as one body receives its warmth from another warm body. Thus, if an animal snuffs up air like this into its nose until such air touches the front of the brain, and alters it to its own odor, the smelling power feels it. As for Taste, it arises only on the coming to pass of the following change: When the moisture of the tasting instrument (organ)--to wit the tongue--becomes transformed into the juice of the newly-come food; and when the mass of this instrument (organ) has received that juice, the tasting power will perceive what has happened within the instrument.