In Phaedo, Socrates puts forth the claim that everything that comes to be comes to be from its opposite. The idea is that all things come into being from their opposites, since nothing can spontaneously come into being or cease to be. If death, for example, is the opposite of life, then death and life must be in a constant cycle, one coming into being out of the other. At the end of our lives we become dead, but analogously this means that at the beginning of our lives we come into being from out of an underworld of dead souls. 

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