Hence it is indispensable that there be something in
between-if there were nothing, so far from seeing with greater
distinctness, we should see nothing at all.
We have now explained the cause why colour cannot be seen
otherwise than in light. Fire on the other hand is seen both in
darkness and in light; this double possibility follows necessarily
from our theory, for it is just fire that makes what is potentially
transparent actually transparent.
The same account holds also of sound and smell; if the object of
either of these senses is in immediate contact with the organ no
sensation is produced. In both cases the object sets in movement
only what lies between, and this in turn sets the organ in movement:
if what sounds or smells is brought into immediate contact with the
organ, no sensation will be produced. The same, in spite of all
appearances, applies also to touch and taste; why there is this
apparent difference will be clear later. What comes between in the
case of sounds is air; the corresponding medium in the case of smell
has no name. But, corresponding to what is transparent in the case
of colour, there is a quality found both in air and water, which
serves as a medium for what has smell-I say 'in water' because animals
that live in water as well as those that live on land seem to
possess the sense of smell, and 'in air' because man and all other
land animals that breathe, perceive smells only when they breathe
air in.
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