Echecrates remarks that he himself sympathizes with those present who thought that the apparent collapse of Socrates' argument indicated that no headway could be made on the subject, and Phaedo replies that Socrates received the young men's argument in a "pleasant, kind and admiring way" (89a) and brought everyone around. Socrates began by discussing with Phaedo himself the importance of not becoming a misologist, or hater of reason. Just as a misanthrope becomes such because he put his trust in someone who failed him, then makes the mistake of generalizing it to all human beings, so too there is a danger that people will find that this or that argument failed them and become misologists by making the error of generalizing it to other arguments indiscriminately.
Socrates makes short work of Simmias's suggestion that the soul is just the harmony of the body by pointing out that he had already agreed with the claim that learning is recollection, which is inconsistent with it. To this he adds the fact that if the soul is a harmony it is governed by its components rather than a governor of them, and also that virtue is a kind of harmony and vice a kind of disharmony, which seems to cause a problem for the idea that a soul could have vices.
Cebes's argument is more difficult, and after making sure he understands what it is, Socrates pauses a while in order to think about it (95e). He decides that the way to address it is to look at the question of generation and corruption, and gives an account of his own intellectual history. When he was young, he was interested in the question of the causes of things, and tried to give material explanations of everything. He came to the conclusion, however, that he had not ability in this kind of inquiry, because all it did was leave him unsure of anything. One day, however, he heard someone read a passage from Anaxagoras, in which that philosopher claimed that "it is Mind that directs and is the cause of everything" (97c) and he realized that it made a certain measure of sense, because then it would be possible to resolve the problems he was having by recognizing that what you are actually trying to do in explanation is not identify the materials, which are merely conditions and not causes, but instead identify what it is that is best. Not looking for what is best results in crude absurdities. He found, however, that Anaxagoras himself, rather than recognize this, simply continued with the material explanations:
This seemed to me much like saying that Socrates' actions are all due to his mind, and then in trying to tell the causes of everything I do, to say that the reason that I am sitting here is because my body consists of bones and sinews, because the bones are hard and are separated by joints, that the sinews are such as to contract and relax, that they surround the bones along with flesh and skin which hold them together, then as the bones are hanging in their sockets, the relaxation and contraction of the sinews enable me to bend my limbs, and that is the cause of my sitting here with my limbs bent. (98c-d)
The problem, of course, is not that any of this is wrong, but that it fails to explain what is to be explained, and leaves out things that are obviously true causes, like the fact that the Athenians condemned him to death and that he refused to flee because he thought it "more right and honorable to endure whatever penalty the city ordered rather than escape and run away" (99a). The error lies in treating a sine qua non condition as if it were properly a cause.
Because of this, Socrates began to focus on logoi, accounts/words/explanations, rather than material things, and came to the conclusion that there are certain things necessary for any account -- the beautiful, the good, the great, and so forth. It is only these that actually explain anything; getting to them, we find we've explained matters, and without them we do not have a true cause but only a condition. Thus he says that it is only through the beautiful (for instance) that things are made beautiful, only through bigness that they are made big and bigger, and so forth.
But of course, this brings us back to contraries in a smooth transition. If Simmias is taller than Socrates but shorter than Phaedo, he seems to be made so by both tallness and shortness; and what it means is that Simmias' shortness is outdone by Phaedo's tallness and Simmias' tallness outdoes Socrates' shortness. But tallness and shortness cannot be mixed in this way. Someone remarks -- Plato, apparently deliberately, leaves out any name -- remarks that this seems to conflict with the original argument from contraries. Socrates points out, however, that there he was talking about things that gain or lose the contraries, and here he is talking about the contraries themselves.
But we notice in dealing with mathematics that even can become odd (for instance, two things can become three things) even though two and three themselves are not opposites in the way even and odd are. Thus if we are talking about bodies, we can go further than just saying that they are made odd by oddness, and say that they are made odd by oneness. And likewise, we do not have to rest content with saying that things are made hot by heat, but can say that they are made hot by that which has heat, e.g., fire.
But if we think about the soul, it is that which, being present, makes something alive. (This is literally all it means in Greek, which is why it is often important not to import our own assumptions about what 'soul' means in to discussion of the Greek word psyche). But if the soul is to life what one is to odd, then it excludes the opposite of life just as one excludes the opposite of odd. The opposite of life is death; so the soul is deathless. But it seems that the deathless must be indestructible; the god, and the Form of Life itself, are indestructible because they utterly exclude death. So, since the soul is deathless, it must be indestructible. When we die, our mortal part dies; but our deathless part stays deathless.
Cebes concedes the argument; Simmias continues to have misgivings, but they have to do not with the argument itself but with his general view that human ability to think in these matters is somewhat weak. Socrates concedes that this is reasonable, and notes that the arguments really do need to be examined more carefully until they become clear -- the implication seems to be to insist that this is fine as long as Simmias is not becoming a misologist.
Assuming that the argument is right, in any case, the care of the soul becomes crucial:
If death were escape from everything, it would be a great boon to the wicked to get rid of the body and of their wickedness together with their soul. But now that the soul appears to be immortal, there is no escape from evil or salvation for it except by becoming as good and wise as possible, for the soul goes to the underworld possessing nothing but its education and upbringing, which are said to bring the greatest benefit or harm to the dead right at the beginning of the journey yonder. (107c-d)
Socrates then transitions into an afterlife myth, in which souls are guided to the underworld by guardian spirits and judged. Those who have been mediocre live on an island in the Acherusian lake, where they are purified of wrongdoing and rewarded for rightdoing; those who have done terrible evils are hurled into Tartarus, where they remain forever if their evil was incurable and for a while if they are curable. Those who have been pious are allowed to ascend and live on the surface of the earth. And there is one more kind of soul:
Those who have purified themselves sufficiently by philosophy live in the future altogether without a body; they make their way to even more beautiful dwelling places which it is hard to describe clearly, nor do we now have the time to do so. Because of the things we have enunciated, Simmias, one must make every effort to share in virtue and wisdom in one's life, for the reward is beautiful and the hope is great. (114c)
Thus we return to where we started: the philosophical life is training for death, because through it one ignores the merely physical things like pleasure and pain and adorns oneself with "moderation, righteousness, courage, freedom, and truth" (115a).
Saying this, Socrates says he wants to take a bath so that the women will not have to wash his corpse. Crito asks if Socrates has any instructions, and Socrates replies that they are the same as always, that they should care for their souls. Crito asks how he wants to be buried, and Socrates laughs, saying that Crito thinks that the real Socrates is the thing that will soon be a corpse, and replies that he can be buried any which way. He bathes, and his students talk while he is gone, then the women and children return and Socrates gives them his last instructions. Then the women are sent away, and since it is now close to sunset, the officer of the Eleven comes to see him, and testifies that he was "the noblest, the gentlest and the best an who has ever come here" (116c).
Socrates prepares to drink the poison, but Crito tries to delay pointing out that the sun is still above the hills, and other men would not hurry; but Socrates says that that is fine for them, but he sees no benefit in waiting. Socrates prays to the gods "that the journey from here to yonder may be fortunate" (117c) and drinks the poison. His students and friends start weeping, and he rebukes them. The poison works slowly and quietly and his last words are to remind Crito not to forget that they owe a cock to Asclepius. Crito closes his mouth and his eyes and the Phaedo, and the series of dialogues devoted to the Last Days of Socrates, reaches its end:
Such was the end of our comrade, Echecrates, a man who, we would say, was of all those we have known the best, and also the wisest and the most upright. (118a)
* The frame narrative breaks into the narrated story at the exact midpoint of the dialogue. Of this, Zuckert says (Plato's Philosophers, p. 766):
The defense Socrates presents of himself and his philosophy in the Phaedo has two parts. In the first part he argues that a philosophical ife is best not merely because it relieves philosophers from the fear of death but also because it results in their possessing the true forms of all the virtues. Socrates' initial arguments seem to depend on the immortality of the soul, however, and he admits that these arguments are open to doubt. So in the second part of his defense Socrates makes himself, his experience, and his central teaching concerning the ideas into the example of the way in which human beings can live with the inescapable ignorance associated with their mortality and the uncertainty to which it gives rise.
However, concern with immortality does not disappear after the break, and thus appears to be much more load-bearing than Zuckert suggests.
* It is worth noting that Socrates' discussion of the best in the causes of generation and corruption gives it a functional role exactly like that of 'necessity' in modern discussions of scientific explanation and the laws of nature.
* Two elements of Socrates' death scene have been hotly debated, and there is still no consensus on them. The first is the hemlock. The symptoms shown are not usually associated with hemlock poisoning, although there are a lot of varieties; so one dispute has been whether the death scene is plausible or simply made up by Plato.
The second is what Socrates means by the reference to owing a cock for Asclepius (the god of healing). Some people take it as a symbolic way of saying that all of life is a sickness. On the other hand, we learn early in the dialogue that Plato is out sick, so perhaps it is for Plato. Or is simply a way of showing that Socrates, condemned for impiety, was pious to the end?
Quotations are from G. M. A. Grube's translation in Plato, Complete Works, Cooper & Hutchinson, eds., pp. 49-100.