 One of the most important philosophical aspects of Kepler’s Astronomia Nova from 1609 (=AN) is its methodological approach and its causal foundation (see Mittelstrass 1972). Kepler was sufficiently conscious of the change of perspective he was introducing into astronomy. Hence, he decided to announce this in the full title of the work: Astronomia Nova, Aitiologetos, seu physica coelestis, tradita commentariis de motibus stellae Martis. Ex observationibus G. V. Tychonis Brahe: New Astronomy Based upon Causes or Celestial Physics Treated by Means of Commentaries on the Motions of the Star Mars from the observations of Tycho Brahe … (trans. Donahue). In the introduction to AN Kepler insists on his radical change of view: his work is about physics, not pure kinematical or geometrical astronomy. “Physics”, as in the traditional, Aristotelian understanding of the discipline, deals with the causes of phenomena, and for Kepler that constitutes his ultimate approach to deciding between rival hypotheses (AN, Engl. trans., p. 48; see Krafft 1991). On the other hand, since his celestial physics uses not only geometrical axioms but also other, non-mathematical axioms, the knowledge obtained often has a kernel of guesswork.