The study of emotions is one of the most interesting, and interdisciplinary topic in contemporary international research, but it is rarely supported by a substantial historical background. This essay aims at reconstructing one of the most significant turning points in the field of affective studies in the seventeenth Century French culture. René Descartes’ last work, Les Passions de l’ âme, proposes an innovative, scientific approach to the study of emotions and their expression, based on a new theory of perception and light. The role played by the visible expression of passions in Descartes’ treatise inaugurates a new and fruitful path of research. Furthermore, the essay analyzes the writings by the painter Charles Le Brun, who proposes a visual interpretation of the expression of emotions, and some works by the physician Marin Cureau de la Chambre, who interprets the passions and their expression from a medical and optical perspective.