Translated by Stephen MacKenna and B. S. Page.
» Contents of this EnneadFIFTH TRACTATE.
PROBLEMS OF THE SOUL (3). [ALSO ENTITLED “ON SIGHT”].
1. We undertook to discuss the question whether sight is possible in the absence of any intervening medium, such as air or some other form of what is known as transparent body: this is the time and place.
It has been explained that seeing and all sense-perception can occur only through the medium of some bodily substance, since in the absence of body the soul is utterly absorbed in the Intellectual Sphere. Sense-perception being the gripping not of the Intellectual but of the sensible alone, the soul, if it is to form any relationship of knowledge, or of impression, with objects of sense, must be brought in some kind of contact with them by means of whatever may bridge the gap.
The knowledge, then, is realized by means of bodily organs: through these, which [in the embodied soul] are almost of one growth with it, being at least its continuations, it comes into something like unity with the alien, since this mutual approach brings about a certain degree of identity [which is the basis of knowledge].
Admitting, then, that some contact with an object is necessary for knowing it, the question of a medium falls to the ground in the case of things identified by any form of touch; but in the case of sight — we leave hearing over for the present — we are still in doubt; is there need of some bodily substance between the eye and the illumined object?
No: such an intervening material may be a favouring circumstance, but essentially it adds nothing to seeing power.
Reference address : https://www.ellopos.net/elpenor/greek-texts/ancient-greece/plotinus/enneads-4b.asp