The Republic of Plato and The Aeneid of Virgil each paint a very different picture of what happens after a person dies, with each description tailored to compliment the rest of the text. The afterlife of The Republic views things in the same black and white, just vs. unjust manner as the rest of the text. The Aeneid , holding with its underlying purpose, uses the underworld as a tool to justify the events of Rome's history, as well as to flatter Virgil's patron, Augustus Caesar.
The main differences between the two texts are in how they treat the souls of the departed. The afterlife of The Republic seems designed to train souls as one would an animal, by rewarding just behavior and punishing unjust in turn. Its overall effect is cyclical, with unjust souls tending to choose more just lives when they are reincarnated, while just souls are made less cautious by their rewards, and have a tendency to choose less just lives for themselves. The afterlife of The Aeneid is less of a place of rewards and retribution and rewards as it is a refinery of sorts, meant to perfect the souls going through it, and to prepare them to be reincarnated as the heroes of the Roman Imperium.
In The Republic the afterlife is represented as two chasms leading down into the ground, and two into the sky. Between the chasms are the judges, who view each soul in turn. Souls that are found to have led a just life are directed onto the right hand road which leads up into the sky, where they will be rewarded for their good deeds for a thousand years. The souls of the unjust are directed onto the left hand road which leads into the earth, and a thousand years of punishment. As there is no middle road, a soul that was only slightly more just than unjust is in for a much better time than one whose soul was judged to be only slightly inclined towards being unjust.
The as the souls are all judged equally based on their deeds, so are they punished and rewarded. "For every wrong he has done anyone a man must pay the penalty in turn, ten times for each" . Those who were judged to have led a just life are rewarded in the same fashion for each good deed they have done. The tenfold rewards and punishments are spread out over the course of a thousand years, at which point the souls exit through the second of each pair of chasms and gather to be reincarnated. The only exception to this is in the case of the souls whose deeds in life were found to be so horrific that they are judged irredeemable. When these souls attempt to leave the underworld at the end of the thousand years, they are instead hurled into the depths of Tartarus where they are damned to an eternity of suffering. There is no greater fear for those souls leaving the underworld then the fear that they will not be allowed to pass.
Once the souls who have spent a thousand years in either torment or bliss have gathered they are brought before the three Fates, Lachesis, Clotho, and Atropos. It is explained to them that they shall be given a choice of lives to be born into, and that this choice shall determine their fate. On the ground at their feet is then cast " ...the different patterns of life, far more in number than the souls who were to choose them. They were of every conceivable kind animal and human." This is the point in where the lessons of the text and the description of the underworld come together, as "If we take all this into account and remember how the soul is constituted, we can choose between the worse life and the better, calling the one that leads us to become more unjust the worse and the one that leads us to be more just the better." Thus is Plato's description of the afterlife shown to be created to compliment the teachings of the rest of the text.
The afterlife of The Aeneid is more similar to the traditional underworld of Greek literature. The reason for this is because while The Republic seeks to reject and replace the teachings of the Greek poets, The Aeneid seeks to build upon their teachings. When a soul dies in The Aeneid it makes its way to the banks of the River Styx, which cannot be crossed without the help of Charon, the boatman. Charon will only take across the souls of those whose bodies have received a proper burial, all others are stranded on its banks. The area on the other side of the Styx is guarded by Cerberus, the three headed hound. This is the final resting place for the souls of those who died as infants, were wrongfully sentenced to death, an those who committed suicide. After these unhappy souls are passed, the newly arriving souls come to a fork in the road. The good souls are directed onto the right hand path, which leads them to the Elysian fields, while the wicked must take the left hand path into Tartarus.
The souls traveling to Tartarus are brought before Rhadamanthus, king of Cnossus who forces them to confess all of their earthly sins, so that even crimes that went unpunished in life are atoned for in death. After they are finished Tisiphone "leaps upon the guilty and flogs them till they writhe, waving fearful serpents over them in their left hand and calling up the cohorts of her savage sisters, the Furies" It is not enough for one to merely have been bad in one's life to be sent to Tartarus, it is reserved for the worst of the worst those who "have all dared to attempt some monstrous crime against the gods, and have succeeded in their attempt" . Those deemed worthy of eternal damnation are punished accordingly and uniquely, each subjected to their own personal hell.
The souls sent to the Elysian fields are rewarded in as diverse a manner as their damned counterparts in Tartarus. The fields are a land of densely wooded groves, where "the spirits have their own sun and their own stars" , and may spend their time as they see fit. There are no fixed homes in the Elysium, as the souls simply wander where they will, and rest where they will. As simply being mostly evil is not enough to be damned for eternity, being mostly good does not ensure one a direct path to the Elysian fields. First all souls must pass through a form of Purgatory in which they atone for their sins, and in doing so are cleansed of them. Once all sins have been atoned for, the purified soul may continue on to the Elysian fields. Unlike Tartarus, a visit to the Elysian fields is not necessarily for an eternity. In fact the majority of souls only spend a thousand years there before being sent back to the world of the living. as with The Republic it is this returning of the souls to life that brings the description of the underworld in line with the rest of the text, for it is as Aeneas observes the souls waiting for reincarnation that Virgil introduces the readers to the great Roman heroes of history and legend, and also takes the time to speak of the glory to come for Augustus Caesar, who happens to be the one who paid Virgil to write the text.
There are some similarities between the afterlives described in the text, but they can for the most part be attributed to tradition and common sense. That in both texts the right hand path is the one that leads to heaven, while the left hand path is the one leading to hell can be attributed to a superstition that has been around for millennia, and was still believed for much of the previous century. The word "sinister" even comes from the Roman word for left handed. The other main similarity is the soul's drinking from the river Lethe to wipe clean their memories before they are reincarnated. This is necessary, otherwise there would be infants being born with memories of being Odysseus himself.
In conclusion, it is obvious that the use of the afterlife in the two texts is as little more than a literary tool to reinforce the theme of the text. Plato uses the strict judgments and rewards to demonstrate that even if it were possible to live an unjust life and be happy, as Thrasymachus claimed, no amount of sacrifices to the gods could protect you from being punished after you die, as the gods themselves have nothing to do with the judging or sentencing of souls, and a man's deeds are all that matters in the end. Virgil uses the afterlife to show that Rome is supported by heaven itself, which is constantly preparing souls to be reborn as Roman citizens and heroes, as well as to show that the glory of Augustus had been foretold centuries before.