De Plantatione

But the Creator made two different races on the earth and in the air. In the air, he made the winged animals capable of being perceived by the external senses, and other powers which can by no means be comprehended in any place

by the external senses; and this is the company of incorporeal souls arranged in order, but not in the same classifications. For it is said that some are assigned to mortal bodies, and are again subjected to a change of place according to certain defined periodical revolutions; but that others which have received a more divinely prepared habitation, look down upon the region of the earth, and that in the highest place, near the other itself, the purest souls are placed, which those who have studied philosophy among the Greeks call heroes, but which Moses, by a felicitous appellation, entitles angels; souls which go as ambassadors and messengers of good from the ruler of all things to his subjects, and messengers also to the king respecting those things of which his subjects have heard. To the earth again he assigned two classes, terrestrial animals and plants, wishing that she should be at the same time their mother and their nurse.