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In Phaedo to argue the soul exists after death with the cyclical argument, Plato posited a philosophical assumption that all opposites —e.g. less, greater; weaker, stronger; sleeping, waking; life, death—are generated out of each other.

One argument for this assumption offered by Socrates is in the quoted text. It tries to prove life is generated from death, just as waking from sleeping. It states if life goes into death but death never back into life, then there will be no life and all things will remain in the state of death.

Is the assumption that all opposites are generated out of each other reasonable?

And that these admissions were not unfair, Cebes, he said, may be shown, I think, as follows: If generation were in a straight line only, and there were no compensation or circle in nature, no turn or return of elements into their opposites, 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 alternation of sleeping and waking, the tale of the sleeping Endymion would in the end have no meaning, because all other things would be asleep too, and he would not be distinguishable from the rest. 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—what other result could there be? For if the living spring from any other things, and they too die, must not all things at last be swallowed up in death?

-- Phaedo 72

BTW, a quite similar question is asked on this site about why the cyclical argument fails, but it didn't consider the above quoted argument, so I don't think it's a duplication here.

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The problem is that Plato fails to make a distinction that Aristotle later discovered.

Some times a thing comes into existence that didn't exist beforehand, like when I form a pot out of some clay. (Call this generation) Other times, a thing that already exist undergoes a change (call this alteration) like when I take a pot that is not currently red and paint it red.

Now in order for an alteration to happen, it looks like we have to have something like what Plato says--the pot "becomes" red when I paint it at 3pm if and only if it was the opposite, i.e. non-red before 3pm, otherwise it wouldn't be "becoming" red at all.

The problem is that Plato thinks this model of change coming about by opposites is the only model of change, but he is neglecting the difference between generation and alteration. He is trying to say that the soul must still exist when the body is dead because there has to exist something x which is dead at 2:59pm and alive at 3:00pm because change comes about through opposites.

However, this does not follow. Consider the case of making the pot out of clay. The pot doesn't exist at 2:59, but it does at 3:00 (suppose I'm very quick at making pots). How is that possible? Is it the case that:

  • (A) that the pot pops into existence out of nothing?

or

  • (B) that there must have already been some pot that had the property of non-existence at 2:59 and gained the property of existence at 3:00?

No, says Aristotle. The pot comes to be out of the clay. The pot doesn't pop into existence out of nowhere, but neither does it simply gain or lose existence. Rather, what Aristotle says is that the lump of clay at 2:59 is potentially, but not actually a pot. Compare the clay with a cubic meter of empty space. The empty space is also not a pot, but unlike the clay the empty space is not even potentially a spot. In short, there doesn't have to be some actually existing dead person from which I come to be when I become alive. Rather, there just has to be some material stuff which is potentially, but not actually alive.

Hope that helps.

  • +1 for your informative answer. But when I considered this question again, I got confused in the very first step -- What is opposite? -- the question is raised here on this site. Would you like to have a look on it and share us your opinion there? – wang zhihao Jan 7 '14 at 7:08

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