De Opificio Mundi

In the first place therefore, from the model of the world, perceptible only by intellect, the Creator made an incorporeal heaven, and an invisible earth, and the form of air and of empty space: the former of which he called darkness, because the air is black by nature; and the other he called the abyss, for empty space is very deep and yawning with immense width. Then he created the incorporeal substance of water and of air, and above all he spread light, being the seventh thing made; and this again was incorporeal, and a model of the sun, perceptible only to intellect, and of all the light-giving stars, which are destined to stand together in heaven.

And air and light he considered worthy of the pre-eminence. For the one he called the breath of God, because it is air, which is the most life-giving of things, and of life the causer is God; and the other he called light, because it is surpassingly beautiful: for that which is perceptible only by intellect is as far more brilliant and splendid than that which is seen, as I conceive, the sun is than darkness, or day than night, or the intellect than any other of the outward senses by which men judge (inasmuch as it is the guide of the entire soul), or the eyes than any other part of the body.

And the invisible divine reason, perceptible only by intellect, he calls the image of God. And the image of this image is that light, perceptible only by the intellect, which is the image of the divine reason, which has explained its generation. And it is a star above the heavens, the source of those stars which are perceptible by the external senses, and if any one were to call it universal light he would not be very wrong; since it is from that the sun and the moon, and all the other planets and fixed stars derive their due light, in proportion as each has power given to it; that unmingled and pure light being obscured when it begins to change, according to the change from that which is perceptible only by the intellect, to that which is perceptible by the external senses; for none of those things which are perceptible to the external senses is pure.

Moses is right also when he says, that darkness was over the face of the abyss. For the air is in a manner spread above the empty space, since having mounted up it entirely fills all that open, and desolate, and empty place, which reaches down to us from the regions below the moon.