A great secret of success is to go through life as a man who never gets used up.

The saying frames “success” less as a single achievement than as sustained vitality—an ability to keep one’s moral energy, curiosity, and capacity for work from being exhausted by routine, disappointment, or complacency. “Never gets used up” suggests a person who continually renews purpose and inner resources, resisting the deadening effects of habit and cynicism. Read in light of Schweitzer’s broader ethical outlook, it implies that endurance and self-renewal are practical virtues: to remain effective in service, one must protect the springs of motivation and compassion. The line also hints that longevity of contribution—staying “fresh” in spirit—is itself a form of success.