This paper explains how Kepler in his ``War onMars'' applied systems of models organized bothin a perspectival and in a stratifiedconceptual sense. With the help of thesesystems Kepler worked out successively moredeterminate models for the planetary orbits.Along the way he discovered the Keplerian lawsas consequences of the distance rule, hisleading regulative principle. The selection ofdecisive, so called privileged, observations,as well as the determinate geometrical andkinematical description of the phenomena,result from the application of this principleto the developing of models. Kepler's method (...) ofdiscovery may therefore appropriately bedesignated converse abduction. (shrink)