On a night in October 1761, in the southern French city of Toulouse, a young man named Marc-Antoine Calas was found dead in his family's shop. His family were Protestants in a Catholic kingdom where Protestant worship was illegal. A rumor spread that his father had killed him to stop him converting to Catholicism. There was no real evidence, but in March 1762 Jean Calas, a sixty-three-year-old merchant, was tortured and executed by being broken on the wheel. He protested his innocence to the end. The case reached an aging writer living near the Swiss border, and he turned it into the most famous campaign of the century. That writer was Voltaire.

A career of provocation

Voltaire was the pen name of François-Marie Arouet, born in Paris in 1694. His wit got him into trouble early. He spent nearly a year in the Bastille for verses mocking the regent, and after a quarrel with a nobleman he went into exile in England from 1726 to 1728. He returned an admirer of English toleration and science. Over a long life he wrote plays, poems, histories, satires, and an enormous correspondence. From 1759 he lived at Ferney, an estate just inside France and a short ride from Geneva, which let him escape across a border if either government came for him.

Lisbon and Candide

On 1 November 1755, All Saints' Day, a massive earthquake struck Lisbon, followed by a tsunami and fires. Tens of thousands died, many of them in churches during Mass. The disaster shook a popular philosophical optimism, associated with Leibniz and the poet Pope, which held that this is the best of all possible worlds and that whatever is, is right. Voltaire answered with an angry poem and then, in 1759, with Candide, a short, savagely funny novel. Its hero and his tutor Pangloss suffer war, shipwreck, earthquake, the Inquisition and slavery, while Pangloss insists all is for the best. The book ends with a modest piece of advice: we must cultivate our garden, meaning do useful work rather than spin grand theories that explain away suffering.

Crush the infamous thing

From the 1760s Voltaire signed many letters with the slogan Écrasez l'infâme, crush the infamous thing. The target was not belief in God, which Voltaire kept in the form of deism, the idea of a creator known through reason and nature rather than revelation. The target was fanaticism and persecution, above all when churches used the power of the state to punish dissent. Voltaire had his own prejudices, including hostile and ugly writing about Jews, which complicates his reputation as a champion of tolerance.

The Calas affair

Voltaire investigated the Calas case, interviewed family members, and became convinced that Marc-Antoine had taken his own life. He published pamphlets, wrote letters to anyone with influence, and brought the case before the public of all Europe. In 1763 he published his Treatise on Tolerance. In 1765 a royal tribunal in Paris overturned the verdict and cleared Jean Calas's name. It came too late for Calas, but it showed that public opinion, mobilized by writers, could force even a royal court system to admit an error.

Words he never said

The line most often attributed to Voltaire, I disapprove of what you say, but I will defend to the death your right to say it, does not appear in his writings. It was written in 1906 by the English author Evelyn Beatrice Hall, as a summary of his attitude in her book The Friends of Voltaire. It is a fair summary, but checking a famous quotation is itself an Enlightenment habit.

Voltaire took up other cases too. He campaigned for the Sirven family, Protestants accused of murdering a daughter, and for the memory of the Chevalier de La Barre, a young nobleman executed in 1766 partly for failing to remove his hat as a religious procession passed and for owning forbidden books, among them Voltaire's own Philosophical Dictionary. When Voltaire returned to Paris in 1778, after almost thirty years away, crowds mobbed his carriage and a performance of his last play turned into a public celebration of him. He died weeks later. In 1791 revolutionaries moved his remains to the Panthéon.

Worked example: running a campaign for justice

How did one writer, with no official position, get a royal verdict reversed? Break the Calas campaign into steps.

• Investigate: Voltaire questioned the Calas sons and studied the trial before taking a public position.
• Make it human: he told the story through a grieving widow and her children, not through abstract arguments about doctrine.
• Use every channel: pamphlets, letters to ministers and nobles, and appeals to foreign readers built pressure from many directions.
• Connect the case to a principle: the Treatise on Tolerance turned one injustice into an argument against religious persecution in general.
• Persist: the campaign took about three years.

The method still works for journalists, lawyers and activists today: check the facts, tell a human story, and link it to a principle people already hold.