Full Idea
All soul is immortal, for what is always in motion is immortal. Only that which moves itself never ceases to be in motion.
Clarification
'Soul' is the Greek word 'psuché', which covers mind and consciousness and life
Gist of Idea
Soul is always in motion, so it must be self-moving and immortal
Source
Plato (Phaedrus [c.368 BCE], 245c)
Book Reference
Plato: 'Phaedrus and Letters VII and VIII', ed/tr. Hamilton,Walter [Penguin 1973], p.49