How much does the traditional understanding of purgatory owe to Plato? Consider this selection quoted by Eusebius in Praeparatio Evangelica (ch. 38):
For [Plato] speaks as follows in the dialogue Concerning the Soul:
‘…As soon as the dead have arrived at the place to which each is conveyed by his genius, first of all they undergo a trial, both those who have lived good and holy and just lives, and those who have not. And those who are found to have led tolerable lives proceed to Acheron, and embarking on such vessels as there are for them, they arrive on board these at the lake; and there they dwell, and by undergoing purification and suffering punishment for their evil deeds they are absolved from any wrongs they have committed, or receive rewards for their good deeds, each according to his deserts. But any who are found to be incurable by reason of the greatness of their sins, having either perpetrated many great acts of sacrilege, or many nefarious and lawless murders, or any other crimes of this kind—-these are hurled by their appropriate doom into Tartarus, whence they never come forth.
‘But those who are found to have committed sins which are great though not incurable, as for instance if in anger they have done any violence to father or mother, and passed the rest of their life in penitence, or have committed homicide in any other similar way, these must also be thrown into Tartarus, but after they have been thrown in and have continued there a year, they are cast out by the wave, the homicides by way of Cocytus, and the parricides by way of Pyriphlegethon: and when they arrive all on fire at the Acherusian lake, there with loud cries they call upon those whom they either slew or outraged; and having summoned them they intreat and beseech them to let them come out into the lake, and to receive them kindly: and if they persuade them, they come out, and cease from their troubles; but if not, they are carried again into Tartarus, and thence back into the rivers, and never have rest from these sufferings, until they have won over those whom they wronged; for this was the sentence appointed for them by the judges.
‘But any who are found to have been pre-eminent in holiness of life—-these are they who are set free and delivered from these regions here on earth, as, from prison-houses, and attain to the pure dwelling place above, and make their abode upon the upper earth. And of this same class those who have fully purified themselves by philosophy live entirely free from troubles for all time to come, and attain to habitations still fairer than these, which it is neither easy to describe, nor does the time suffice at present. But for the sake of these things which I have described we ought, Simmias, to make every effort to gain a share of virtue and of wisdom in our lifetime: for fair is the prize, and great the hope.’