If the transmigration of a soul takes place into a rational being, it simply becomes the soul of that body. But if the soul migrates into a brute beast, it follows the body outside, as a guardian spirit follows a man. For there could never be a rational soul in an irrational being.

XX. On Transmigration of Souls, and how Souls are said to migrate into brute beasts. - On the Gods and the Cosmos