The Phaedo begins when Echecrates asks Phaedo to tell him about Socrates' death, and Phaedo warmly welcomes the chance to remember his friend Socrates in the final hours of his life. He says that it was an astonishing experience because although he was witnessing the death of a dear friend, he had no pity because of the way in which Socrates bravely and happily faced it, unafraid of the unknown. He takes care to mention that Plato was not there due to illness and further explains the course of the day to Echecrates.
In the days prior to Socrates' death, Phaedo and other friends frequently visited Socrates in jail. Each time they discuss something new and interesting, including Socrates' thoughts on why it is wrong to kill one's self. Socrates thinks that it shows disrespect to the gods when one chooses to commit suicide because men are possessions of the gods and the gods are right to be angry if one of their possessions kills himself. He believes that only after the gods make it necessary (as his situation is of necessity, for example) is it permissible to choose death over life. And Cebes thinks that what Socrates has said doesn't make sense because the wise should always want to be with better people than himself so he can learn from them, as philosophers want to do. If one dies, one will not be with wiser people anymore and so the wise should resent death while the foolish should rejoice at it. Socrates then tries to defend his argument by showing Cebes and Simmias that if death is the separation of the soul from the body, the philosopher by being a philosopher seeks to have himself separated as much as possible from his body (because the body inhibits the mind). And therefore, the philosopher practices death everyday in his life. So the philosopher, as an example of "the wise" would not resent death as Cebes originally thought.
The men then take up discussion on what exactly death is, and Socrates tells us a lot of the qualities of the soul in the opinion of Plato. He says that the soul is capable of reasoning and thought, and that those capacities alone can grasp relevant objects, so he believes that no thought at all can be reached through the senses of the body. This is why Plato was considered to be an a priori philosopher, which means that he thought knowledge could be attained completely independent of experience. This is a comforting thought for someone like Socrates who is close to death. If he cares about knowledge (which we know that he does) he certainly has no cause to fear dying since it will in his belief, free his soul from his body so it can better learn and move closer to knowledge.
A little later in the text, Cebes tells Socrates that everything he has said so far has been excellent. But, "I think, men find it hard to believe what you said about the soul. They think that after it has left the body it no longer exists anywhere, but that it is destroyed and dissolved on the day the man dies,". He asks Socrates to speak a little to this fact and Socrates accepts the challenge, beginning by a discussion on the immortality of the soul.
There are two arguments that Socrates relies on to support his claim. They are the following:
1a) Argument from recollection
1b) Argument from affinity
The first argument means that the soul existed before birth, and as Socrates explains, it was then that the soul learned all that it could learn from the Forms. The Forms are discussed at length in Plato's Republic, but in summary they are intangible, divine, perfect, ideals of things such as Beauty, Good, Truth, etc. and all things in the world derive their existence and identity from the Forms. At birth, all that was previously learned is forgotten and that is why we start out knowing so little, if anything at all. We grow in life and learn more and more by "recollecting" our prior knowledge, until we can arrive at true belief which is one step below knowledge. Only in death can we again arrive at the knowledge that we possessed before birth, when our souls were communing with the Forms, as they are again in death.
The second part of the first argument is that our souls exist after death. Socrates attempts to show this to be true by indirectly constructing a logical proof. Assume that opposites come from opposites. For example, wake comes from sleep. Then assume that life is the opposite of death. We can then conclude that life comes from death. Secondly, assume that one existed before some time "x". Then assume that one's body did not exist before some time "x". We can then conclude that one does not equal one's body. And this gives support to the claim that our soul and body are two different entities. If you combine both of these proofs with the other assumptions that Socrates gives, namely that before life one's body did not exist, yet one did exist before life, so one is either one's soul or body and since the body clearly dies after death we must be our souls, which means that we were dead souls before we were alive. This proves to Simmias and Cebes that the soul exists after death, and therefore the soul must be immortal.
The second argument that Socrates makes is for the existence of the Forms. This argument is not quite as clear, but Socrates thinks that when the soul abandons the body in death it passes on into a realm of intangible, divine, perfect, ideals that describe the Forms. Although he never uses the term "Forms" we can assume that this was what Plato was trying to convey based on textual evidence in other works.
After satisfying his friends with the previous discussion, Socrates talks at length about the different kinds of souls that different people have. The philosophers soul is in the most desirable state because philosophers spend their lives contemplating the Forms and by doing this they gain wisdom, much more so than the average man or woman. They are compelled as Socrates says, by the pursuit of wisdom, and therefore try to rely only on their minds since the body and senses can be deceiving. At some point in the conversation, Simmias and Cebes admit that they still have some doubt about whether one can have knowledge about what happens to the soul after it separates from the body, as in death. Socrates is pleased to hear that his friends need more clarification because he knows that shortly he will be apart from them, and their conversation is more valuable now than ever.
Simmias says that the previously mentioned problem is analogous to a lyre and its harmony. If the lyre is the tangible matter, similar to the body, then the harmony is the intangible perfect music similar to the soul. He wonders what happens to the music when someone breaks the lyre and Socrates answers that the soul is actually not analogous to the harmony because harmony cannot exist prior to the things from which it was created from, and that goes against the entire argument that the soul did indeed exist before the body, which Simmias previously agreed. Socrates further explains that the Forms of the Good and Beautiful, etc. are causes of things, and that the soul is immortal. Beautiful things participate in the Form of Beauty. That is, a statue of Helen of Troy is considered beautiful by people because it derives its beautiful qualities from the Form of Beauty.
Socrates then uses Simmias as an example to introduce the idea of "carriers". If Simmias is both taller than Socrates but shorter than Phaedo, then be exhibits both tallness and shortness. The problem is that tallness is obviously not the same as shortness and it cannot be both tall and short as in the case of Simmias' tallness. If one quality won't admit of another then it either flees when its opposite is present or is destroyed by its opposite. "Carriers" are things that share the same properties of qualities as in the example of fire and heat. Heat will not admit of cold (because they are opposites) just as fire will not admit of water. Fire and water are carriers of heat and cold, respectively. All of this is analogous to the soul according to Socrates. He thinks now that he has adequately proven that the soul is that which makes the body live, so it can't admit of death. Therefore the soul is deathless, or immortal.
The Phaedo ends with a Socrates saying that since the soul is immortal, it requires our care in life by being good so that we may fare well in death. He believes, or rather it is more appropriate to say that Plato believes that your life in the underworld and in the next life depends on the state of your soul. Obviously philosophers were considered by Plato to have the best souls because they spent their lives in search of knowledge and therefore they can make better choices and have a better chance of choosing a good life, all because of the knowledge that they acquired and the way in which they lived their lives.
Finally, Socrates drinks the hemlock and his friends become quite upset. Socrates scolds them slightly and says that he sent the women away for fear of them reacting with tears. Phaedo says that he felt ashamed for crying when Socrates was so dignified, and then the great philosopher and man took his last breath of air before dying. Those who knew him held him best held him in such high regard at the very end of his life, and at perhaps the most poignant time in his life-his death.