Plato on Purgatory

I was reading some Plato earlier this summer and just happened to come across something that sounded as if it could be from the council of Trent. (Speaking of the afterlife from the Phaedo) "Those who appear to have lived neither well nor ill, go to the river of Acheron, and embarking in any vessels which they may find, are carried in them to the lake, and there they dwell and are purified of their evil deeds, and have suffered the penalty of the wrongs which they have done to others, they are absolved, and recieve the rewards of their good deeds, each of them according to his desserts." It seems that purgatory has it's origins in Greek philosophy rather than scripture. I guess that's what happens when human tradition is given as much authority as the word of God.