Metempsychosis

Metempsychosis, the transmigration of souls, holds that the soul is immortal and is reborn into a succession of bodies. The doctrine is associated above all with Pythagoras and the Pythagoreans, who extended a kind of kinship to all living things and so abstained from harming them; a famous anecdote has Pythagoras stop a man from beating a dog, claiming to recognize in its cry the voice of a departed friend.

In Plato

Plato takes it up in several dialogues, each adding a vision of the soul's fate. The Myth of Er that closes the Republic shows souls between lives choosing their next existence and drinking from the river of Forgetfulness before rebirth. The Phaedo argues for the soul's immortality and prior existence, and the Phaedrus and Timaeus tie the soul's next life to how it lived this one — the philosophic soul rising toward the gods, the base or bestial soul sinking into lower forms.

The moral stakes

The cycle of rebirth gives weight to the present life and frames philosophy as purification — the soul's effort to free itself from the body and break the round of return. Through recollection, traces of former vision persist across lives, and in Neoplatonism the ascent of the soul is, in part, an escape from the wheel of becoming.