The soul through all her being is immortal, for that which is ever in motion is immortal; but that which moves another and is moved by another, in ceasing to move ceases also to live.
Plato’s Phaedrus grounds the soul’s immortality in the logical argument that self-motion, being its own principle, can neither begin nor cease, making the self-moving soul necessarily deathless.
, Phaedrus, -370thesis