The paper try to provide a contribution to the scientific—historiographic debate concerning the relations between experiments, metaphysics and mathematics in Descartes’ physics. The three works on which the analysis is focused are the Principia philosophiae and the two physical essays: La Dioptrique and Les Météores. The authors will highlight the profound methodological and epistemological differences characterizing, from one side, the Principia and, from the other side, the physical essays. Three significant examples will be dealt with: 1) the collision rules in the Principia philosophiae; 2) the refraction law in La Dioptrique; 3) the rainbow in Les Météores. In the final remarks these differences will be interpreted as depending upon the different role Descartes ascribed to the three books inside his whole work. The concepts of intensity and gradation of the physical quantities used by Descartes will provide an important interpretative means. In this paper, we compare the aprioristic approach to physics typical for Descartes’ Principia with the experimental and mathematical one characterizing Descartes’ Essays.
