"Then look at it this way, Cebes, and you'll see, I think, that our admissions were not mistaken. If there were not perpetual reciprocity in coming to be, between one set of things and another, revolving in a circle, as it wereif, instead, genesis were a linear process from one thing into its opposite only, without any bending back in the other direction or reversal, do you realize that all things would ultimately have the same form: the same fate would overtake them, and they would cease from coming to be?"
"What do you mean?"
"It's not at all hard to understand what I mean. If, for example, there were such a thing as going to sleep, but from sleeping there were no reverse process of waking up, you realize that everything would ultimately make Endymion seem a mere trifle: he'd be nowhere, because the same fate as his, sleeping, would have overtaken everything else. Again, if everything were combined, but not separated, then Anaxagoras' notion of "all things together" would soon be realized. And similarly, my dear Cebes, if all things that partake in life were to die, but when they'd died, the dead remained in that form, and didn't come back to life, wouldn't it be quite inevitable that everything would ultimately be dead, and nothing would live? Because if the living things came to be from the other things, but the living things were to die, what could possibly prevent everything from being completely spent in being dead?"
Plato, Phaedo (1993:20)