Phaedo

exhalation of the air rising and falling as the waters pass into the depths of the earth and return again, in their course forming lakes and rivers, but never descending below the centre of the earth; for on either side the rivers flowing either way are stopped by a precipice. These rivers are many and mighty, and there are four principal ones, Oceanus, Acheron, Pyriphlegethon, and Cocytus. Oceanus is the river which encircles the earth; Acheron takes an opposite direction, and after flowing under the earth through desert places, at last reaches the Acherusian lake,--this is the river at which the souls of the dead await their return to earth. Pyriphlegethon is a stream of fire, which coils round the earth and flows into the depths of Tartarus. The fourth river, Cocytus, is that which is called by the poets the Stygian river, and passes into and forms the lake Styx, from the waters of which it gains new and strange powers. This river, too, falls into Tartarus. The dead are first of all judged according to their deeds, and those who are incurable are thrust into Tartarus, from which they never come out. Those who have only committed venial sins are first purified of them, and then rewarded for the good which they have done. Those who have committed crimes, great indeed, but not unpardonable, are thrust into Tartarus, but are cast forth at the end of a year by way of Pyriphlegethon or Cocytus, and these carry them as far as the Acherusian lake, where they call upon their victims to let them come out of the rivers into the lake. And if they prevail, then they are let out and their sufferings cease: if not, they are borne unceasingly into Tartarus and back again, until they at last obtain mercy. The pure souls also receive their reward, and have their abode in the upper earth, and a select few in still fairer 'mansions.…