Key here is the discussion of the afterlife, particularly proof that the soul exists after death, and is immortal. In Crito, one of the critical reasons for living a good life was that a person would be treated well in the afterlife. This means the pains of this life can be ignored or mitigated by the rewards to come. If there are no rewards after this world, there is no reason for restraint.
Cebes lays out the problem when he talks about the skepticism most people have about the immortality of the soul.
when she leaves the body her place may be nowhere, and that on the very day of death she may be destroyed and perish-immediately on her release from the body … if she could only hold together and be herself after she was released from the evils of the body, there would be good reason to hope …
But much persuasion and many arguments are required in order to prove that when the man is dead the soul yet exists, and has any force of intelligence.
This means Plato needs to find proof that the soul is immortal, and in The Phaedo, he runs through a set that seem convincing.
Socrates begins with the proposition, “Are not all things which have opposites generated out of their opposites?”, meaning “is there not an opposite of life, as sleep is the opposite of waking?” Death is the obvious answer – so far, so good; nothing to object to here.
He then leads Cebes neatly into a logical impasse
“What is generated from life?
And what from death?
I can only say in answer-life.
The rest of the loose ends are quickly tied up. If death is the release of the soul, birth must be the return of the souls of the dead into the world of the living. After all, “if all things which partook of life were to die, and after they were dead remained in the form of death, and did not come to life again, all would at last die, and nothing would be alive – how could this be otherwise? ”
One of the first proofs is recollection, or preknowledge. When we have an idea of something that does not exist insofar as it cannot be detected by the senses – truth, equality, beauty, etc. – where does it come from? Socrates’ answer is that we had it when we were born, lose it at birth and afterwards, remember it again.
Affinity (rejecting the body)
“The soul”, claims Socrates, “clings to that which itself resembles, the invisible, the divine, immortal and wise”; all those things that are the opposite of the body. And these things, or forms, are unchanging, they are the opposite of the physical world where a person can possess beauty, then lose it or be just one day and unjust the next.
The lovers of wisdom know that philosophy, receiving their soul plainly bound and glued to the body … gently exhorts it, and endeavors to free it.
This is such a necessary task, because “each pleasure and pain, having a nail, as it were, nails the soul to the body, and fastens it to it”, and when the soul and the body become close
the soul too is then dragged by the body into the region of the changeable, and wanders and is confused; the world spins round her, and she is like a drunkard when under their influence … but when returning into herself she reflects; then she passes into the realm of purity, and eternity, and immortality, and unchangeableness, which are her kindred.
For Socrates, it is this affinity of the soul with eternal things that signifies its immortality. In the same way that cold can never allow hot to approach it (because then it would not be cold) or an even number can never be uneven, the soul is the thing that brings the body to life, and when death approaches the body, the soul will withdraw to preserve its integrity.
The moral argument
To round things off, and to bring a precursor of Christianity, there is also a moral perspective. And it’s an appealing one.
If death were a deliverance from everything, it would be a great gain for the wicked, when they die, to be delivered at the same time from the body and from their vices, together with the soul; but now, since it appears to be immortal, it can have no other refuge from evils, nor safety, except by becoming as good and wise as possible.
This moral aspect ties together Socrates’ thinking nicely. there is a moral imperative to what we’re doing. In becoming a Socratic philosopher, there is a sacrifice of the body and pleasures, and this needs to have a definite reward.
Acting in opposition to the prevailing norms of society, whether it’s being a human gadfly or turning the other cheek becomes much easier if we are shown to be better than society because of that action. Or rather, not only that we are the better person, but that someone will recognise us as the better person, and reward us for it – not even in the most selfless world is anything done without thought of a reward.
Nothing is more appealing to have yourself vindicated, although drinking hemlock is a hard way of going about it.