 A general presentation of Kepler’s philosophical attitude and principles is not complete without reference to his link to the world of experience. For, despite his mainly theoretical approach in the natural sciences, Kepler often emphasized the significance of experience and, in general, of empirical data. In his correspondence there are many remarks about the significance of observation and experience, as for instance in a letter to Herwart von Hohenburg from 1598 (KGW 13, let. N° 91, lines 150–152) or from 1603 to Fabricius (KGW 14, let. N° 262, p. 191, lines 129–130), to mention only two of his most important correspondents. Looking for empirical support for the Copernican system, Kepler compares different astronomical tables in his MC, and in AN he makes extensive use of Tycho’s observational treasure trove. In MC (chapter 18) he quotes a long passage from Rheticus for the sake of rhetorical support when, as was the case here, the data of the tables he used did not fit perfectly with the calculated values from the polyhedral hypothesis. In this passage, the reader learns that the great Copernicus, whose world system Kepler defends in MC, said one day to Rheticus that it made no sense to insist on absolute agreement with the data, because these themselves were surely not perfect. After all, it is questionable whether Kepler, using for instance the Prutenic Tables (1551) of Erasmus Reinhold (1511–1553), had access to complete and correct empirical information to confirm the Copernican hypothesis in grand style, as he claimed (for an analysis of Reihold’s tables and their influence see Gingerich 1993, pp. 205–255).