Aulus Gellius (ancient Roman author and grammarian) (c. 125 – after 180 AD)
Book 5, Chapter 16
On the function of the eye and the process of vision.
1 I have observed that the philosophers have varying opinions about the method of seeing and the nature of vision. 2 The Stoics say that the causes of sight are the emission of rays from the eyes to those objects which can be seen, and the simultaneous expansion of the air. 3 Epicurus believes that there is a constant flow from all bodies of images of those bodies themselves, and that these impinge upon the eyes and hence the sensation of seeing arises. 4 Plato is of the opinion that a kind of fire or light issues from the eyes, and that this, being united and joined either with the light of the sun or with that of some other fire, by means of its own and the external force makes us see whatever it has struck and illumined. 5 But here too we must not dally longer, but follow the advice of that Neoptolemus in Ennius, of whom I have just written, who advises having a “taste” of philosophy, but not “gorging oneself with it.”