What does it mean when Plato says "If, for example, there was such a process as going to sleep, but no corresponding process of waking up, you realize that in the end everything would show the story of Endymion to have no meaning….In the same way, my dear Cebes, if everything that partakes of life were to die and remain in that state and not come to life again, would not everything ultimately have to be dead and nothing alive? Even if the living came from some other source, and all that lived died, how could all things avoid being absorbed in death" in Pheado?