A Parisian physicist, mathematician and philosopher, Jean le Rond d'Alembert was one of the most important figures in the French Enlightenment. He studied philosophy, law and fine arts. In 1738 he became a lawyer. Later he became interested in medicine and mathematics, achieving significant results especially in the latter field. In 1741 he entered the Académie des Sciences and four years later the Berlin Academy. In 1746 he met Denis Diderot (1713-1784), whose most important collaborator he was to become, starting in 1758, in the complex editorial activity of the Encyclopédie ou dictionnaire raisonné des sciences, des arts et des métiers. The theoretical basis of the work was established expressly in the famous "Preliminary Discourse" of d'Alembert which appeared at the beginning of the first volume in 1751. The fundamental scope of the work was the rigourous, organic construction of the tree of knowledge. Its contributions were mainly of scientific nature.