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.