d’Alembert Jean Le Rond (1717–83), French mathematician, philosopher, and Encyclopedist. According to Grimm, d’Alembert was the prime luminary of the philosophic party. An abandoned, illegitimate child, he nonetheless received an outstanding education at the Jansenist Collège des Quatre-Nations in Paris. He read law for a while, tried medicine, and settled on mathematics. In 1743, he published an acclaimed Treatise of Dynamics. Subsequently, he joined the Paris Academy of Sciences and contributed decisive works on mathematics and physics. In 1754, he was elected to the French Academy, of which he later became permanent secretary. In association with Diderot, he launched the Encyclopedia, for which he wrote the epoch-making Discours préliminaire (1751) and numerous entries on science. Unwilling to compromise with the censorship, he resigned as coeditor in 1758. In the Discours préliminaire, d’Alembert specified the divisions of the philosophical discourse on man: pneumatology, logic, and ethics. Contrary to Christian philosophies, he limited pneumatology to the investigation of the human soul. Prefiguring positivism, his Essay on the Elements of Philosophy (1759) defines philosophy as a comparative examination of physical phenomena. Influenced by Bacon, Locke, and Newton, d’Alembert’s epistemology associates Cartesian psychology with the sensory origin of ideas. Though assuming the universe to be rationally ordered, he discarded metaphysical questions as inconclusive. The substance, or the essence, of soul and matter, is unknowable. Agnosticism ineluctably arises from his empirically based naturalism. D’Alembert is prominently featured in D’Alembert’s Dream (1769), Diderot’s dialogical apology for materialism. See also ENCYCLOPEDI. J.-L.S.