The waters of the River Lethe are said to wash clean the memories of the recently deceased. This process returns them to a tabula rasa state of mind, after which point their souls can migrate to new bodies. What is not clear from myth alone, however, is the mechanism by which the Lethe's waters perform this function. The living often assume that these memories simply dissolve into the imbibed fluid, as though they were merely salt.
What the dead know, and the living do not, is that these memories pass from their immaterial selves as urine. Though the waters of the Lethe can flow through the mind as easily as the body, they must eventually drain away, as all other waters do. The soul can resist this inevitability for as long as they like, but the longer they do so, the more painful it becomes. Eventually, they must allow every moment, no matter how beautiful or painful, to be discarded as filth before they can progress to the next phase of their afterlife.
This ritual takes place inside a colossal, nondescript cube of polished porcelain, into which the dead wander single file, and leave in the same manner. It is the only known access point to the sewers of heaven, which run deeper than the underworld known to the gods, who have chosen not to acknowledge the existence of such things. It is an entire world unto itself, one of living bile and knots of black lead, profane things sealed beneath a nameless wonder. Likewise, human beings have struck this domain from their own mythology, not just because of disgust, but also out of embarrassment for the imperfection that it implies of their own spirits.
Even harder for these mortals to accept, however, is that this is all the material world is to the divine: grime to be washed clean from reality, then drained into saturnine pipes. For those unprepared, the horror of this realization is often more than they can bear, but it never lasts long; everyone eventually gives in to relief.