Full Idea
As far as the literature tells us, Pherecydes of Syros was the first who pronounced the souls of men to be eternal.
Gist of Idea
Pherecydes was the first to say that the soul is eternal
Source
report of Pherecydes (fragments/reports [c.600 BCE]) by M. Tullius Cicero - Tusculan Disputations I.xvi.38
Book Reference
Cicero: 'Tusculan Disputations', ed/tr. King,J.E. [Harvard Loeb 1927], p.45
A Reaction
Presumably before that it was the physical person who arrived in the Underworld. The Hindu tradition seems to require the soul to be very long-lived, if not eternal. Why did Pherecydes come up with this idea?