The argument from contraries in Phaedo begins with the unobjectionable claim that whatever comes to be X was non-X before, or that things come to be from opposites; but then Plato follows this with a remarkable premise:
[Opposites] involve a passage into and out of one another. And this holds of all opposites, even though not always expressed in words-they are generated out of one another, and there is a passing or process from one to the other of them?
That is, every opposite gives rise to its opposite. The claim seems crazy: moving to Paris certainly involves coming to it from the outside, but the move isn’t part of some larger cycle of moving into Paris and then out of it; growth involves getting larger, but (jokes about Grandma notwithstanding) there is no opposite process of becoming smaller. There is not just a obvious problem of various examples of coming to be, but the ontological problem of saying that a thing by nature gives rise to its opposite – which seems to mean that a thing seeks its own negation. So what could Plato have been thinking?
If generation were in a straight line only, and there were no compensation or circle in nature, no turn or return into one another, then you know that all things would at last have the same form and pass into the same state, and there would be no more generation of them.
What do you mean? he said.
A simple thing enough, which I will illustrate by the case of sleep, he replied. You know that if there were no compensation of sleeping and waking, the story of the sleeping Endymion would in the end have no meaning, because all other things would be asleep, too, and he would not be thought of. Or if there were composition only, and no division of substances, then the chaos of Anaxagoras would come again. And in like manner, my dear Cebes, if all things which partook of life were to die, and after they were dead remained in the form of death, and did not come to life again, all would at last die, and nothing would be alive-how could this be otherwise? For if the living spring from any others who are not the dead, and they die, must not all things at last be swallowed up in death?
So he shifts to a fundamental level of nature, and claims that death cannot swallow up all life.