Book excerpt: Voltaire's Treatise on Toleration is one of the most important essays on religious tolerance and freedom of thought A powerful, impassioned case for the values of freedom of conscience and religious tolerance, Treatise on Toleration was written after the Toulouse merchant Jean Calas was falsely accused of murdering his son and executed on the wheel in 1762. As it became clear that Calas had been persecuted by 'an irrational mob' for being a Protestant, the Enlightenment philosopher Voltaire began a campaign to vindicate him and his family. The resulting work, a screed against fanaticism and a plea for understanding, is as fresh and urgent today as when it was written.