The longer he lives the more he discovers that real peace depends on the strength with which he rules his own heart, and real security depends on the truth with which he rules his own mind. When he leaves his emotions in disorder they bring agony--as the accompaniment or the follower of the happiness they claimed at first to be able to give. When he lets his thoughts serve the blindnesses of his ego, they deceive, mislead, or trouble him.
-- Notebooks Category 6: Emotions and Ethics > Chapter 3: Discipline Emotions > # 21
-- Perspectives > Chapter 6: Emotions and Ethics > # 13