 Beyond his own original talent, it is clear that Kepler was trained in mathematics from his earliest studies at Tübingen. At least officially, his positions at Graz, Prague, Linz, Ulm and Sagan can be characterized as the typical professional occupations of a mathematician in the broadest sense, i.e., including astrology and astronomy, theoretical mechanics and pneumatics, metrology, and every topic that could in some way be related to mathematics. Besides the field of astronomy and optics, where mathematics is ordinarily applied in different ways, Kepler produced original contributions to the theory of logarithms and above all within his favorite field, geometry (especially with his stereometrical investigations. For the medieval proportions theory as a background for Kepler's logarithms, see Rommevaux-Tani, 2018). Thus, on account of his natural predilection and talent and the importance of mathematics, particularly of geometry, for his thought, it is not surprising to find many different passages in his works where he articulated his philosophy of mathematics. However, Kepler’s principal exposition on this topic is to be found in his HM, a work in which the first two books are purely mathematical in content. As he himself declares, in HM he played the role “not of a geometer in philosophy but of a philosopher in this part of geometry” (KGW 6, p. 20, Engl. trans., p. 14).