The Calas Affair and the Campaign Against Intolerance

In October 1761 Marc-Antoine Calas, son of a Protestant merchant in Toulouse, was found dead in the family home. Rumors spread that his father, Jean Calas, had killed him to stop him from converting to Catholicism, and in March 1762 the Toulouse parlement had the elderly father executed despite thin evidence. Voltaire, then living at Ferney near Geneva, investigated and became convinced that the son had taken his own life and that religious prejudice had driven the verdict.

He launched a public campaign of letters, pamphlets and his Treatise on Tolerance (1763), mobilizing European opinion. In 1765 royal judges declared Jean Calas innocent. The affair showed Voltaire turning philosophy into activism, under his slogan 'Écrasez l'infâme', crush the infamous thing, meaning fanaticism and church-backed persecution. It also helped create the idea that public opinion could act as a court of appeal against injustice.