On the night of November 17, 1717, Mme Claudine-Alexandrine Guérin, Marquise de Tencin, gave birth to a son whom she promptly abandoned on the steps of the Church of Saint-Jean-Le-Rond. There, he was baptized with the name of the church; he was then sent to the Maison de la Coucher, from which he went to a foster home in Picardy. When his father, Louis-Camus Destouches, a military officer, returned to Paris, he sought his son and arranged for the child to be cared for by Mme Rousseau, the wife of a glazier. D’Alembert would always regard Mme Rousseau as his real mother and would continue to live with her until 1765, when illness compelled him to seek new quarters in the home of Julie de Lespinasse.
Destouches continued to watch over his illegitimate child, sending him to private schools; when Destouches died in 1726, he left the boy a legacy of twelve hundred livres a year. The sum, though not luxurious, guaranteed him an independence he cherished throughout his life. Through the interest of the Destouches family, the young man entered the Jansenist Collège des Quatre-Nations, where he took the name Jean-Baptiste Daremberg, later changing it, perhaps for euphony, to d’Alembert. Although he, like many other Enlightenment figures, abandoned the religious training he received there, he never shed the Cartesian influence that dominated the school.
After receiving his baccalauréat in 1735, he spent two years studying law, receiving a license to practice in 1738. Neither jurisprudence nor medicine, to which he devoted a year, held his interest. He turned to mathematics, for which he had a natural talent. At the age of twenty-two, he submitted his first paper to the Académie des Sciences; in that piece, he corrected a number of errors in Father Charles Reyneau’s Analyse demontrée (1714). A second paper, on refraction and fluid mechanics, followed the next year, and in May, 1741, he was made an adjunct member of the Académie des Sciences.