If [as by the theory of an intervenient] the percipient mind or soul remains within itself and needs the light only as one might need a stick in the hand to touch something at a distance, then the perception will be a sort of tussle: the light must be conceived as something thrusting, something aimed at a mark, and similarly, the object, considered as an illuminated thing, must be conceived to be resistant; for this is the normal process in the case of contact by the agency of an intervenient.
Besides, even on this explanation, the mind must have previously been in contact with the object in the entire absence of intervenient; only if that has happened could contact through an intervenient bring knowledge, a knowledge by way of memory, and, even more emphatically, by way of reasoned comparison [ending in identification]: but this process of memory and comparison is excluded by the theory of first knowledge through the agency of a medium.
Finally, we may be told that the impinging light is modified by the thing to be seen and so becomes able to present something perceptible before the visual organ; but this simply brings us back to the theory of an intervenient changed midway by the object, an explanation whose difficulties we have already indicated.