What the expression is intended to mean, I think, is that there is a better and a worse element in the character of each individual, and that when the naturally better element controls the worse then the man is said to be "master of himself", as a term of praise. But when - as a result of bad upbringing or bad company one s better element is overpowered by the numerical superiority of one s worse impulses, then one is criticized for not being master of oneself and for lack of self control.
PlatoO youth or young man, who fancy that you are neglected by the gods, know that if you become worse, you shall go to worse souls, or if better to the better... In every succession of life and death, you will do and suffer what like may fitly suffer at the hands of like. This is the justice of heaven.
PlatoExcess of liberty, whether it lies in state or individuals, seems only to pass into excess of slavery.
Plato