On the Giants (8-9)

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.