"And when the angels of God saw the daughters of men that they were beautiful, they took unto themselves wives of all of them whom they chose." Genesis vi. 2. Those beings, whom other philosophers call demons, Moses usually calls angels; and they are souls hovering in the air. And let no one suppose, that what is here stated is a fable, for it is necessarily true that the universe must be filled with living things in all its parts, since every one of its primary and elementary portions contains its appropriate animals and such as are consistent with its nature; —the earth containing terrestrial animals, the sea and the rivers containing aquatic animals, and the fire such as are born in the fire (but it is said, that such as these last are found chiefly in Macedonia), and the heaven containing the stars: for these also are entire souls pervading the universe, being unadulterated and divine, inasmuch as they move in a circle, which is the kind of motion most akin to the mind, for every one of them is the parent mind.
It is therefore necessary that the air also should be full of living beings. And these beings are invisible to us, inasmuch as the air itself is not visible to mortal sight. But it does not follow, because our sight is incapable of perceiving the forms of souls, that for that reason there are no souls in the air; but it follows of necessity that they must be comprehended by the mind, in order that like may be contemplated by like. Since what shall we say? Must we not say that these animals which are terrestrial or aquatic live in air and spirit? What? Are not pestilential afflictions accustomed to exist when the air is tainted or corrupted, as if that were the cause of all such assuming vitality? Again, when the air is free from all taint and innocent, such as it is especially wont to be when the north wind prevails, does not the imbibing of a purer air tend to a more vigorous and more lasting duration of life?