The hypothesis that the light contiguous to the visible object is affected, and transmits this affection by relays from point to point into the eye, is essentially identical with that theory which supposes that the medium must be preliminarily modified by the visible object; a hypothesis that has already been discussed above. NEITHER FOR HEARING IS THE AIR NECESSARY AS A MEDIUM. 5. As to hearing, there are several theories. One is that the air is first set in motion, and that this motion, being transmitted unaltered from point to point from the (location of the) sound-producing air as far as the ear, causes the sound to arrive to the sense. Again, another theory is that the medium is here affected accidentally, and only because it happens to be interposed; so that, if the medium were annihilated, we would feel the sound immediately on its production by the shock of two bodies. We might think that the air must first be set in motion, but the medium interposed (between the first moved air and the ear) plays a different part. The air here seems to be the sovereign condition of the production of sound; for, at the origin of the sound, the shock of two bodies would produce no sound if the air, compressed and struck by their rapid concussion did not transmit the motion from point to point as far as the ear.[173] But if the production of the sound depend on the impulsion impressed on the air, the (qualitative) difference between voices and (instrumental) sounds will challenge explanation; for there is great difference (of "timbre") between metal struck by metal of the same kind, or another. These differences are not merely quantitative, and cannot be attributed to the air which (everywhere) is the same, nor to the force of the stimulus (which may be equal in intensity). Another theory (of Aristotle's) is that the production of voices and sound is due to the air, because the impulsion impressed on the air is sonorous. (To this it should be answered that) air, in so far as it is air, is not the cause of sound; for it resounds only in so far as it resembles some solid body, remaining in its situation, before it dilates, as if it were something solid.[174] The (cause of the sound) then is the shock between objects, which forms the sound that reaches the sense of hearing. This is demonstrated by the sounds produced in the interior of animals, without the presence of any air, whenever one part is struck by some other. Such is the sound produced by certain articulations when they are bent (as, the knee); or certain bones, when they are struck against each other, or when they break; in this case air has nothing to do with the production of the sound. These considerations compel a theory of hearing similar to our conclusions about sight. The perception of audition, like that of vision, therefore consists in a repercussion (an affection sympathetically felt) in the universal organism. THE RELATION OF THE AIR TO THE LIGHT.