2. If vision[166] presupposes the union of the "light of the eye,"[167]
with the light interposed (between the eye) and the sense-object
itself, the interposed medium is the light, and this medium is
necessary, on this hypothesis. (On the theory of Aristotle) the colored
substance produces a modification in the medium; but nothing here
would hinder this modification from reaching the eye itself, even
when there is no medium. For, in this case, the medium is necessarily
modified before the eye is. (The Platonic philosophers) teach that
vision operates by an effusion of the light of the eye. They have no
need to postulate a medium, unless indeed they should fear that the
ray of the eye should lose its way; but this ray is luminous, and
the light travels in a straight line. (The Stoics) explain vision by
the resistance experienced by the visual ray. They cannot do without
a medium.[168] (The Atomists and) the believers in "images" (such
as Epicurus), insist that these images move in emptiness, thereby
implying the existence of a free space to avoid hindering the images.
Consequently as they will be hindered in a direct ratio to the
existence of a medium, this opinion does not run counter to our own
hypothesis (that there is no medium).
A COSMOLOGICAL MEDIUM IS NECESSARY, BUT IT AFFECTS SIGHT ONLY
ACCIDENTALLY.
Those who (with Plotinos himself) teach that vision operates by
sympathy, assert that vision is poorer through a medium, because this
medium hinders, fetters, and weakens sympathy. In this case, indeed,
the medium necessarily weakens sympathy even though it shared the
same nature (as the eye and the object), and was affected in the same
manner. (It acts like the integument) of some body that is deeply
burned by fire applied to it; the interior parts are less affected
because they are protected by the exterior parts. There is no doubt
that the parts of one and the same animal will be less affected in
experiencing sympathy because of the existence of a medium. The
affection will be weakened according to the nature of the medium,
because such a medium would hinder excess of affection, unless indeed
that which is transmitted (by one part to another) is not such as to
fail to affect the medium. But if the universe sympathize with itself
because it constitutes a single organism, and if we are affected
because we are contained within this single organism, and form part of
it, why should any continuity be necessary for us to feel a distant
object? The single organism, indeed, could not be continuous without
the continuity of some medium; this continuous medium is affected only
by accident; but otherwise we would have to admit that all can be
affected by all. But if these two objects are affected in one manner,
and other two objects are affected in another manner, there might not
always be need of a medium. Whoever asserts the need of a medium for
vision will have to advance a very good argument, inasmuch as that
which traverses the air does not always affect the air, and often
limits itself to dividing the air. Thus when a stone falls the only
thing that happens to the air is that it fails to support the stone.
As falling is part of the stone's nature, it would be unreasonable to
assert that its falling was due to the reaction exerted by the ambient
air. Otherwise we would have to assert that it is this same reaction of
the ambient air that makes fire ascend, which is absurd; because the
fire, by the rapidity of its motion, forestalls this reaction. That, by
the very rapidity of the motion, reaction is accelerated, takes place
only by accident, and has no relation to the upward impulsion; for
trees grow from above without receiving any (upward) impulsion. Even
we, when walking, divide the air without being pushed by the reaction
of the air; the air behind us limits itself to filling the void we have
created. If then the air allow itself to be divided by bodies without
being affected by them, what would hinder the air from permitting free
transit for the images to reach the eye, without being thereby divided?