[53] [cap.17] The aforesaid number therefore being accounted worthy of such pre-eminence in nature, the Creator of necessity adorned the heaven by the number four, namely by that most beautiful and most godlike ornament the light-giving stars. And knowing that of all existing things light is the most excellent, he made it the instrument of the best of all the senses, sight. For what the mind is in the soul, that the eye is in the body. For each of them sees, the one beholding those existing things which are perceptible only to the intellect, and the other those which are perceptible to the external senses.
But the mind is in need of knowledge in order to distinguish incorporeal things, and the eyes have need of light in order to be able to perceive bodies, and light is also the cause of many other good things to men, and particularly of the greatest, namely philosophy.