All soul is immortal. For that which is always in movement is immortal; that which moves something else, and is moved by something else, in ceasing from movement ceases from living. So only that which moves itself, because it does not abandon itself, never stops moving. But it is also source and first principle of movement for the other things which move.

Phaedo: Translated, with Introd., Notes, and Appendices (ed. 1955)