What is true by lamplight is not always true by sunlight.

Joseph Joubert (1754–1824) was a French moralist and essayist known less for published books than for the notebooks and pensées he kept throughout his life. Many of his best-known remarks circulated posthumously, edited and issued by friends and literary heirs, and later translated into English in various anthologies. This aphorism fits Joubert’s recurring concern with the limits of perception and judgment: how mood, partial knowledge, or the “lighting” of circumstance can make an idea seem convincing in one moment and dubious in another. It reflects the salon and reflective culture of late Enlightenment/early Romantic France, where brief maxims were a favored vehicle for moral psychology.

The line suggests that what seems true under dim, artificial, or private conditions (“lamplight”) may not withstand fuller illumination (“sunlight”). Joubert uses a physical metaphor for intellectual and moral scrutiny: in the half-light we mistake outlines, overlook flaws, and let imagination fill gaps; in daylight, details and contradictions appear. The aphorism can be read as advice to test convictions—especially those formed in solitude, passion, fatigue, or secrecy—against clearer evidence, broader perspective, and public reason. It also implies humility: our certainties are often products of conditions rather than stable realities, so we should revisit judgments when circumstances change.