 Johannes Kepler (1571–1630) is one of the most significant representatives of the so-called Scientific Revolution of the 16th and 17th centuries. Although he received only the basic training of a “magister” and was professionally oriented towards theology at the beginning of his career, he rapidly became known for his mathematical skills and theoretical creativity. As a convinced Copernican, Kepler was able to defend the new system on different fronts: against the old astronomers who still sustained the system of Ptolemy, against the Aristotelian natural philosophers, against the followers of the new “mixed system” of Tycho Brahe—whom Kepler succeeded as Imperial Mathematician in Prague—and even against the standard Copernican position according to which the new system was to be considered merely as a computational device and not necessarily a physical reality. Kepler’s complete corpus can be hardly summarized as a “system” of ideas like scholastic philosophy or the new Cartesian systems which arose in the second half of the 17th century. Nevertheless, it is possible to identify two main tendencies, one linked to Platonism and giving priority to the role of geometry in the structure of the world, the other connected with the Aristotelian tradition and accentuating the role of experience and causality in epistemology. While he attained immortal fame in astronomy because of his three planetary laws, Kepler also made fundamental contributions in the fields of optics and mathematics. To the little-known facts regarding Kepler’s indefatigable scientific activity belong his efforts to develop different technical devices, for instance, a water pump, which he tried to patent and apply in different practical contexts (for the documents, see KGW 21.2.2., pp. 509–57 and 667–691). To his contemporaries he was also a famous mathematician and astrologer; for his own part, he wanted to be considered a philosopher who investigated the innermost structure of the cosmos scientifically.