Jean Le Rond d'Alembert

Jean Le Rond d'Alembert was a French mathematician, physicist, philosopher, and music theorist. He co-edited the *Encyclopédie* alongside Denis Diderot until 1759. His best-known contributions to mathematics and physics are the wave equation (often called d'Alembert's equation) and the formula he devised to solve it. In France, the fundamental theorem of algebra bears his name. D'Alembert was born in Paris on 16 November 1717, the illegitimate son of writer Claudine Guérin de Tencin and artillery officer Louis-Camus Destouches. His father was abroad at the time of his birth. A few days later, his mother abandoned him on the steps of the Saint-Jean-le-Rond church in Paris, and he was named after the church's patron saint, as was the custom. Placed in a foundling orphanage, he was soon retrieved by his father, who entrusted him to the wife of a glazier, Madame Rousseau. D'Alembert lived with her for nearly fifty years. She offered him little encouragement; when he told her of a discovery or a piece of writing, she would typically reply that he would never be anything but a philosopher—an ass who torments himself all his life just to be talked about after death. Destouches secretly paid for his son's education but refused to officially acknowledge paternity. D'Alembert began his education at a private school. Upon Destouches' death in 1726, he received an annuity of 1,200 livres. Influenced by the Destouches family, he entered the Jansenist Collège des Quatre-Nations (also called Collège Mazarin) at age twelve. There he studied philosophy, law, and the arts, earning a baccalauréat in arts in 1735. Later in life, he rejected the Cartesian principles the Jansenists had taught him—specifically "physical promotion, innate ideas and the vortices." The Jansenists tried to steer him toward an ecclesiastical career and away from poetry and mathematics, but he found theology "rather unsubstantial fodder." He studied law for two years and was named an avocat in 1738. He also had interests in medicine and mathematics. Initially enrolling as Jean-Baptiste Daremberg, he later changed his name, likely for euphony, to d'Alembert. Frederick the Great of Prussia later proposed naming a suspected (but nonexistent) moon of Venus after d'Alembert, but d'Alembert declined the honor. In July 1739, d'Alembert made his first mathematical contribution by pointing out errors in Charles-René Reynaud's *Analyse démontrée* (published 1708), a standard work he had used to study mathematics. He addressed his findings to the Académie des Sciences. A capable Latin scholar, he spent part of his later life translating Tacitus, earning praise from Denis Diderot. In 1740, he submitted a second scientific work on fluid mechanics, *Mémoire sur la réfraction des corps solides*, which Clairaut recognized; in it, d'Alembert theoretically explained refraction. After several unsuccessful attempts, he was elected to the Académie des Sciences in 1741. He was later elected to the Berlin Academy in 1746 and became a Fellow of the Royal Society in 1748. His most famous work, *Traité de dynamique*, appeared in 1743, where he developed his own laws of motion. When the *Encyclopédie* was organized in the late 1740s, d'Alembert was brought on as co-editor for mathematics and science alongside Diderot, serving until a series of crises halted publication in 1757. He wrote over a thousand articles for the project, including the well-known Preliminary Discourse. D'Alembert "abandoned the foundation of Materialism" when he "doubted whether there exists outside us anything corresponding to what we suppose we see," aligning with the idealist Berkeley and anticipating Kant's transcendental idealism. In 1752, he wrote about what is now called d'Alembert's paradox: the drag on a body in an inviscid, incompressible fluid is zero. He was elected to the Académie Française in 1754 and became its Permanent Secretary on 9 April 1772. In 1757, an article he wrote for the seventh volume of the *Encyclopedia* claimed, based on information from Voltaire, that Geneva's clergymen had moved from Calvinism to pure Socinianism. The pastors were indignant and formed a committee to respond. Under pressure from Jacob Vernes, Jean-Jacques Rousseau, and others, d'Alembert excused himself by saying he meant only that anyone not accepting the Church of Rome was a Socinianist, and he stopped working on the encyclopedia after this response. He was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1781. D'Alembert's first engagement with music theory came in 1749, when he was asked to review a *Mémoire* submitted to the Académie by Jean-Philippe Rameau. The article, co-written with Diderot, later became the basis of Rameau's 1750 treatise *Démonstration du principe de l'harmonie*. D'Alembert praised it highly, seeing in Rameau's deductive, systematic method an ideal scientific model that supported his own ideas. In 1752, he attempted a comprehensive survey of Rameau's works in his *Éléments de musique théorique et pratique suivant les principes de M. Rameau*. He claimed to have "clarified, developed, and simplified" Rameau's principles, arguing that the single idea of the *corps sonore* was not sufficient to derive all of music; three principles were needed for the major mode, minor mode, and identity of octaves. Because he was not a musician, d'Alembert misconstrued finer points of Rameau's thinking, changing and removing concepts that did not fit his understanding. Although initially grateful, Rameau eventually turned on d'Alembert, leading to bitter exchanges and the end of their friendship. D'Alembert also discussed music in his *Discours préliminaire* for the *Encyclopédie*, claiming that music had not been able to represent as much of reality due to "lack of sufficient inventiveness and resourcefulness of those who cultivate it." He praised Rameau as "that manly, courageous, and fruitful genius" who picked up the slack left by Jean-Baptiste Lully. D'Alembert participated in several Parisian salons, particularly those of Marie Thérèse Rodet Geoffrin, the marquise du Deffand, and Julie de Lespinasse. He became infatuated with Julie de Lespinasse and eventually took up residence with her. He suffered bad health for many years and died from a urinary bladder illness. As a known unbeliever, he was buried in a common unmarked grave.