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Château de Cirey

In 1733, Voltaire met Émilie du Châtelet (Marquise du Châtelet), a mathematician and married mother of three, who was 12 years his junior and with whom he was to have an affair for 16 years. To avoid arrest after the publication of Lettres, Voltaire took refuge at her husband's château at Cirey on the borders of Champagne and Lorraine. Voltaire paid for the building's renovation, and Émilie's husband sometimes stayed at the château with his wife and her lover. The intellectual paramours collected around 21,000 books, an enormous number for the time. Together, they studied these books and performed scientific experiments at Cirey, including an attempt to determine the nature of fire.

Having learned from his previous brushes with the authorities, Voltaire began his habit of avoiding open confrontation with the authorities and denying any awkward responsibility. He continued to write plays, such as Mérope (or La Mérope française) and began his long researches into science and history. Again, a main source of inspiration for Voltaire were the years of his British exile, during which he had been strongly influenced by Newton's works. Voltaire strongly believed in Newton's theories; he performed experiments in optics at Cirey, and was one of the promulgators of the famous story of Newton's inspiration from the falling apple, which he had learned from Newton's niece in London and first mentioned in his Letters.

In the fall of 1735, Voltaire was visited by Francesco Algarotti, who was preparing a book about Newton in Italian. Partly inspired by the visit, the Marquise translated Newton's Latin Principia into French, which remained the definitive French version into the 21st century. Both she and Voltaire were also curious about the philosophy of Gottfried Leibniz, a contemporary and rival of Newton. While Voltaire remained a firm Newtonian, the Marquise adopted certain aspects of Leibniz's critiques. Voltaire's own book Elements of the Philosophy of Newton made the great scientist accessible to a far greater public, and the Marquise wrote a celebratory review in the Journal des savants. Voltaire's work was instrumental in bringing about general acceptance of Newton's optical and gravitational theories in France, in contrast to the theories of Descartes.

Voltaire and the Marquise also studied history, particularly the great contributors to civilization. Voltaire's second essay in English had been "Essay upon the Civil Wars in France". It was followed by La Henriade, an epic poem on the French King Henri IV, glorifying his attempt to end the Catholic-Protestant massacres with the Edict of Nantes, which established religious toleration. There followed a historical novel on King Charles XII of Sweden. These, along with his Letters on the English, mark the beginning of Voltaire's open criticism of intolerance and established religions. Voltaire and the Marquise also explored philosophy, particularly metaphysical questions concerning the existence of God and the soul. Voltaire and the Marquise analyzed the Bible and concluded that much of its content was dubious. Voltaire's critical views on religion led to his belief in separation of church and state and religious freedom, ideas that he had formed after his stay in England.

In August 1736, Frederick the Great, then Crown Prince of Prussia and a great admirer of Voltaire, initiated a correspondence with him. That December, Voltaire moved to Holland for two months and became acquainted with the scientists Herman Boerhaave and Willem 's Gravesande. From mid-1739 to mid-1740 Voltaire lived largely in Brussels, at first with the Marquise, who was unsuccessfully attempting to pursue a 60-year-old family legal case regarding the ownership of two estates in Limburg. In July 1740, he traveled to the Hague on behalf of Frederick in an attempt to dissuade a dubious publisher, van Duren, from printing without permission Frederick's Anti-Machiavel. In September Voltaire and Frederick (now King) met for the first time in Moyland Castle near Cleves and in November Voltaire was Frederick's guest in Berlin for two weeks, followed by a meeting in September 1742 at Aix-la-Chapelle. Voltaire was sent to Frederick's court in 1743 by the French government as an envoy and spy to gauge Frederick's military intentions in the War of the Austrian Succession.

Though deeply committed to the Marquise, Voltaire by 1744 found life at her château confining. On a visit to Paris that year, he found a new love—his niece. At first, his attraction to Marie Louise Mignot was clearly sexual, as evidenced by his letters to her (only discovered in 1957). Much later, they lived together, perhaps platonically, and remained together until Voltaire's death. Meanwhile, the Marquise also took a lover, the Marquis de Saint-Lambert.