“He who knows other men is discerning; he who knows himself is intelligent. He who overcomes others is strong; he who overcomes himself is mighty.”
Knowledge of others and external power are contrasted with the harder achievements of self-knowledge and self-mastery, locating a more durable form of strength within.
Source details
Tao Teh King, chapter 33, James Legge translation; the chapter continues by connecting contentment with richness and steadfastness with will.