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?