Nobody can be so amusingly arrogant as a young man who has just discovered an old idea and thinks it is his own.

Harris is poking fun at a familiar rite of passage: the exhilaration of “discovering” an idea for oneself and mistaking that personal revelation for originality. The line targets not youth as such, but the arrogance that can accompany intellectual awakening—when enthusiasm outruns historical awareness. It also hints at a broader cultural truth: many “new” insights are rediscoveries, and maturity often consists in learning the lineage of one’s thoughts. The humor softens the critique while encouraging humility, reading, and attentiveness to predecessors—virtues especially important in public debate and creative work.