The first time in extant work that psuche seems to mean soul, the essential ‘you,’ potentially immortal, is in Heraclitus in the late sixth century… Psuche becomes the vehicle of personal identity between now and any possible afterlife.
Padel traces the genealogy by which psuche shifts from a Homeric ghost-shade to the Platonic locus of personal identity and potential immortality, identifying Heraclitus and Pindar as the pivotal early witnesses.
, In and Out of the Mind Greek Images of the Tragic Self, 1994thesis