Something can come to be in a state opposite a give state (smaller to larger, weaker to stronger) only if it was first in that opposite state. This is an indisputable truth, but it says very little. Socrates adds that it is through the first opposite that the second opposite state comes to be. There are two opposing states of the body: life and death. So if the living body was at first dead, it came to be alive through death. A body comes to life through its inhabitation by a soul, and it dies when the soul no longer inhabits the body. We can explain how the body comes to life through death by the postulate that the soul that gives life had left a body that had come to die. Here is the argument in schematic form.
1. Opposites come to be only from opposites
2. Life is the opposite of death
3. So, life comes to be through death
4. Life can come from death only if the soul already exists without the body
5. The soul exists without the body only due to the death of a previous body
6. So, the soul exists after death
This argument is quite unpersuasive. Step 1 is ambiguous in the way already explained. If we suppose that a body was once dead and now lives, there is a sense in which life came to be from death. But there is no further reason to think that it became alive through its previous state of death. Indeed, later in the dialogue, Plato explains that opposites in fact recede at the approach of the other. Another problem is with the explanation of how life comes to be through death. The soul is said to have survived the death of a previous body to give life to the current one. So what came to be was life in one body through the death in a different body. But this is not what motivates the "opposites come from opposites" principle. Instead, it is the passing of opposite to opposite in the same thing. A child matures to become larger and stronger. But we do not generally explain the child's becoming larger by means of another child's becoming smaller.
