Chapter 33 of the Tao Te Ching reads:
Self-Knowledge & Self-Mastery
Intelligent people know others.
Enlightened people know themselves.
You can conquer others with power,
But it takes true strength to conquer yourself.
Ambitious people force their will on others,
But content people are already wealthy.
Prudent people will abide.
People unconquered by the idea of death will live long.
People who live according to their means last long.
I remain intrigued by the quote "We have conquered outer space, but not inner space," meaning we have achieved technological but not psychological maturity.
Self-mastery is the key to the development of inner peace, and with inner peace as the means, outer peace is the end.