Secular humanism affirms free inquiry, that the search for knowledge and truth must not be constrained by dogma, creed, or authority, and that all beliefs and institutions should be open to questioning, evidence, and revision. Humanists hold that human understanding advances through the liberty to think, investigate, publish, and dissent, and that no doctrine should be shielded from scrutiny. Free inquiry entails freedom of thought and expression and a readiness to follow reason and evidence wherever they lead, even against established opinion.
The classic argument for free inquiry as a positive good, not merely a permitted liberty, comes from John Stuart Mill's On Liberty (1859), which held that even false or mistaken opinions should remain free to be expressed and contested, both because human fallibility means any silenced view might turn out to be true or partly true, and because even a wholly false opinion, when openly challenged, forces the true opinion to be held more vigorously than if it went unchallenged as dead dogma. Mill's argument, developed to defend general civil liberty rather than religious scepticism specifically, has become one of the most frequently cited philosophical foundations for humanist and secularist defences of free inquiry against calls to protect religious or political doctrines from criticism.