Happiness is mental harmony unhappiness is mental inharmony.

Allen’s aphorism frames happiness and unhappiness not as products of external fortune but as inner states determined by the mind’s order. “Mental harmony” suggests coherence among one’s thoughts, desires, and principles—an inward alignment that produces calm and contentment. “Mental inharmony” implies conflict: divided aims, resentful or fearful thinking, or a mismatch between ideals and conduct, which generates distress. The significance lies in its ethical-psychological claim: lasting well-being is cultivated through self-discipline, right thinking, and inner congruence rather than pursued through changing circumstances. It echoes the broader self-help and moral philosophy tradition that treats character and thought as the primary causes of emotional life.