How could someone support the 'argument of opposites' in the Phaedo?
The Argument From Opposites in the Phaedo was one of Socrates' ways of proving the immortality of the soul. In this argument, he says that everything comes to be by having first been its opposite. He gives the example that a small thing must have been big and has since decreased and a big thing must have been small and then increased.
It is not clear to me whether you want support for the idea of opposites or for using that argument to prove the immortality of the soul. I assume, though, that you are trying to do the former.
I would simply support this from examples. You might argue, for example, that day and night are opposites. It is clear for anyone to see that night comes into existence from the day and vice versa.
You could argue that war and peace are opposites. And you can say that at one point you have peace, and at the next, war. This would prove that opposite things come from one another.
I'm not sure I buy it, but that's how I would argue it if I had to.