Chapter XV.

Whether Darkness Can be Visible to Us.

The Stoics¹ 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 forth² which are neither black nor cloudy. Upon this. account darkness is visible to us.

1 stoics   

禁欲主义者，恬淡寡欲的人，不以苦乐为意的人( stoic的名词复数 )

参考例句：

• Stoics I can handle this shit. 斯多葛：我能掌握这大便。
• The most famous exercise of meditation is the premeditatio mallorum as practiced by the Stoics. 冥思最著名的练习是禁欲学派所实行的[消灾冥思]。

2 forth   

adv.向前；向外，往外

参考例句：

• The wind moved the trees gently back and forth.风吹得树轻轻地来回摇晃。
• He gave forth a series of works in rapid succession.他很快连续发表了一系列的作品。