Cebes says that even if the soul preexists and is later born into a new body, time and again, it might still eventually perish. Socrates has an argument in response to this challenge by Cebes. He starts with the principle that no opposite which remains the same can ever be or become its own opposite, so that, for example, greatness cannot become smallness. Then he takes a look at the things that contain opposites which remain the same. We are told that the number 3 contains oddness.
Oddness is an essence.
It cannot become its opposite, evenness. But notice also, says Socrates, that the number 3 cannot become even or contain evenness. In a similar fashion, the soul contains life. Just as oddness cannot become evenness, so, too, life cannot become death. And just as 3 cannot become even or contain evenness, so, too, the soul cannot become dead or contain death.