Plotinos: Complete Works, v. 2 In Chronological Order,...

2. If vision166 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).