It is the natural duty of souls to do their work in the body; are we to suppose that when once they leave the body they spend all eternity in idleness? Again, if the souls did not again enter into bodies, they must either be infinite in number or God must constantly be making new ones. But there is nothing infinite in the world; for in a finite whole there cannot be an infinite part. Neither can others be made; for everything in which something new goes on being created, must be imperfect. And the world, being made by a perfect author, ought naturally to be perfect.

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