Cebes agrees with much of what Socrates has said regarding the soul, but does not feel convinced that the soul coheres and remains active and intelligent after death. For instance, he suggests, when the soul leaves the body, it may be dissipated like breath or smoke so that it no longer exists as one coherent unit.
Socrates' answer begins with a consideration of the myth that the soul exists in some other world after death, and that after some time it returns to animate another body in this world. If this is true, Socrates suggests, then the soul must cohere after death, since otherwise it could not return to animate another body. Socrates' task, then, will be to show that the souls of the dead may return to this world in other bodies.
Here Socrates introduces the Argument from Opposites. He puts forth the claim that everything that comes to be, comes to be from its opposite. For instance, for an object to become bigger, it must have been smaller beforehand, and has become bigger out of this smallness. Further, there are two forms of generation between opposites, where each opposite comes into being out of the other opposite. For instance, between big and small there are the twin processes of increase and decrease.
Socrates then drives his point home by asking Cebes whether or not there is an opposite to living. Cebes replies that being dead is opposite to living. From this claim and the Argument from Opposites, it follows that dead things go from being living to being dead through the process of dying, and that similarly, living things must go from being dead to being living through the process of coming to life.
Socrates also notes that if this were not the case, soon all the world would be dead. That is, if all living things died, but new living things were not made from those that had died, the number of dead would soon very quickly supercede and overwhelm the number of the living. If the living could only be made out of other living beings, there would only be a limited stock of living beings before they all run out.
The Argument from Opposites is relatively easy to understand in itself, though it is couched in some Presocratic philosophy which might be unclear and has a number of problems that should be discussed. The bulk of this commentary will cover those two questions, though a quick clarification of the argument might be in order before we begin. 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 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.