Translated by Stephen MacKenna and B. S. Page.
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2. If sight depends upon the linking of the light of vision with the light leading progressively to the illumined object, then, by the very hypothesis, one intervening substance, the light, is indispensable: but if the illuminated body, which is the object of vision, serves as an agent operating certain changes, some such change might very well impinge immediately upon the eye, requiring no medium; this all the more, since as things are the intervening substance, which actually does exist, is in some degree changed at the point of contact with the eye [and so cannot be in itself a requisite to vision].
Those who have made vision a forth-going act [and not an in-coming from the object] need not postulate an intervening substance — unless, indeed, to provide against the ray from the eye failing on its path — but this is a ray of light and light flies straight. Those who make vision depend upon resistance are obliged to postulate an intervening substance.
The champions of the image, with its transit through a void, are seeking the way of least resistance; but since the entire absence of intervenient gives a still easier path they will not oppose that hypothesis.
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