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.
The two Presocratic schools of thought that are present in this section are the Pythagorean school, of which Cebes, Simmias, Phaedo, and Echecrates are all members, and the Heraclitean school, named for its founder Heraclitus. The Pythagoreans were noted physicists, and held that nothing in the universe comes to be or ceases to be spontaneously. Apparent generation and corruption are just surface changes in an unchanging, underlying substance. Thus, the Pythagoreans would be eager to agree with Plato that nothing is ultimately created or destroyed at birth or death, but that rather there is just a change where opposites come into being out of one another. Heraclitus also maintained that things come to be out of their opposites, and that the balance between opposites is crucial for the existence of order in the universe. He also maintains that everything is in constant flux and that the only constant in the universe is change. Plato is resisting this extreme, arguing not only for the constancy of the soul, but also of his unchanging Forms.
The problems in this section arise from the shifting question of what Plato means when he discusses opposites. As he first introduces them, it seems he is talking about comparative opposites, such as bigger and smaller. Something that becomes big is bigger now than it was before: before it was smaller. It is presently big only in comparison to what it was before. But then he shifts from comparative opposites to absolute opposites. There is no discussion of "more dead" or "more alive," there is just "dead" or "alive" to choose between. Someone that is alive now is not said to be alive because they are less dead than they were when they weren't alive. There are no gradations of being dead or alive as there are with being big or small, and it is less clear that in the case of absolute opposites, each one must come into being out of its opposite.
There is also the question of what is dead and alive. For Socrates' argument to work, he must conclude that the soul is what goes through these states of change, thereby proving that it does cohere even after death. But is it the soul that dies? Surely, Socrates wants to argue that the body dies while the soul lives on, free from the body. But if it is the body that dies, the argument breaks down. It would be absurd to suggest that living bodies come into being from dead bodies, and besides, this would not help us to establish the immortality of the soul. Perhaps Socrates might have chosen the opposing pair "embodied"/"disembodied" to describe the progress of the soul between its opposites, rather than "alive"/"dead."
Finally, we might consider Nietzsche's words: "Life is not the opposite of death; it is an exception to death, and a very rare one at that."