Chapter Xv. Whether Darkness Can Be Visible to Us.

The Stoics say that darkness is seen by us, for out of our eyes there issues out some light into it; and our eyes do not impose upon us, for they really perceive there is darkness. Chrysippus says that we see darkness by the striking of the intermediate air; for the visual spirits which proceed from the principal part of the soul and reach to the ball of the eye pierce this air, which, after they have made those strokes upon it, extend conically on the surrounding air, where this is homogeneous in quality. For from the eyes those rays are poured forth which are neither black nor cloudy. Upon this account darkness is visible to us.