One must always hope when one is desperate, and doubt when one hopes.

The aphorism captures a psychological paradox: in extremity, hope becomes a survival mechanism—less a rational forecast than a necessary stance that keeps a person moving. By contrast, when circumstances invite optimism, Flaubert suggests that skepticism is the wiser companion, guarding against self-deception and the complacency that hope can breed. The pairing implies a disciplined emotional economy: hope is most “useful” when it counters despair, while doubt is most “useful” when it tests comforting narratives. Read this way, the line reflects Flaubert’s broader suspicion of easy consolations and his preference for lucidity over sentimentality.