Keller argues that beliefs should be assessed partly by the consequences they produce in everyday life. Rather than treating optimism and pessimism as purely private attitudes, she considers how each may influence behaviour, effort and social development. If optimism encourages people to act, cooperate and pursue improvement, while pessimism discourages such efforts, she suggests that their practical effects become relevant when judging their value.

The passage therefore links philosophical belief with responsibility for its consequences. Keller does not simply claim that an idea is true because it produces desirable results; her emphasis is on the importance of examining what beliefs encourage people to do. Her warning about pessimism reflects this practical standard: a philosophy that consistently weakens constructive action may have wider effects beyond the individual who holds it. The quote presents ideas as forces capable of shaping both personal conduct and collective progress.