Aulus Gellius, V.XVI:
On the Power of the Eye and the Mechanics of Vision
“We note that there are many diverse opinions held by philosophers concerning the mechanics of vision and the nature of discerning things. The Stoics say that sight is caused by an emission of rays from the eyes unto those things which are seen, in conjunction with an expansion of the air. Epicurus says that certain likenesses of the material objects themselves flow out from those objects, and that they bring themselves into the eyes and thus vision occurs. Plato thinks that there is a certain type of fire and light which comes from the eyes and that this combines with the light from the sun or some other fire, and this conjunction of its own and the external force makes it so that we see whatever it comes upon and illuminates. But we shouldn’t dilly-dally here any longer; we should make use of the precept of that same Neoptolemus of Ennius whom I wrote earlier, who thought that philosophy should be enjoyed in a tiny little taste, and not one big swill.”
De vi oculorum deque videndi rationibus.
1 De videndi ratione deque cernendi natura diversas esse opiniones philosophorum animadvertimus. 2Stoici causas esse videndi dicunt radiorum ex oculis in ea, quae videri queunt, emissionem aerisque simul intentionem. 3 Epicurus afluere semper ex omnibus corporibus simulacra quaedam corporum ipsorum eaque sese in oculos inferre atque ita fieri sensum videndi putat. 4 Plato existimat genus quoddam ignis lucisque de oculis exire idque coniunctum continuatumque vel cum luce solis vel cum alterius ignis lumine sua vi et externa nixum efficere, ut, quaecumque offenderit inlustraveritque, cernamus. 5 Sed hic aeque non diutius muginandum, eiusdemque illius Enniani Neoptolemi, de quo supra scripsimus, consilio utendum est, qui degustandum ex philosophia censet, non in eam ingurgitandum.