If the light which is contiguous to the eye should become animated,
and if the soul should, so to speak, interpenetrate it, uniting with
it as she unites with the interior light, there would be no need
of intermediary light for the perception of the visible object.
Sight resembles touch; it operates in light by somehow transferring
itself to the object, without the medium experiencing any affection.
Now consider: does the sight transfer itself to the visible object
because of the existence of an interval between them, or because
of the existence of some body in the interval? In the latter case,
vision would occur by removing this obstacle. If, on the other hand,
it be because of the existence of a mere interval, then the nature
of the visible object must seem inert and entirely inactive. This is
however impossible; not only does touch announce and experience the
neighboring object but, by the affection it experiences, it proclaims
the differences of the tangible object, and even perceives it from
a distance, if nothing oppose it; for we perceive the fire at the
same time as the air that surrounds us, and before this air has been
heated by the fire. A solid body heats better than does the air; and
consequently it receives heat through the air, rather than by the
intermediation of air. If then the visible object have the power to
act, and if the organ have the power of experiencing (or suffering),
why should sight need any intermediary (besides light) to exert its
power? This would really be needing an obstacle! When the light of the
sun reaches us, it does not light up the air before lighting us, but
lights both simultaneously; even before it has reached the eye, while
it is still elsewhere, we have already seen, just as if the air was
not affected at all; that is the case, probably, because the medium
has undergone no modification, and because light has not yet presented
itself to our view. Under this hypothesis (which asserts that the air
receives and transmits an affection) it would be difficult to explain
why during the night we see the stars and, in general, any kind of fire.