Montesquieu, one of the major political philosophers of the Enlightenment, author of the famous De l’esprit des lois (1748), left copious marginal notes in a Cartesian textbook published by a mathematics teacher, Nicolas Guisnée, and entitled Application de l’algèbre à la géométrie (Paris 1705). Some years later, young d’Alembert studied and commented upon the same text, writing a continuous commentary, which is still unpublished. The focus of the talk will be on different reading practices of Montesquieu and d’Alembert, their motivations and the goals they might have pursued.