The Fate of Kepler's Philosophical Thought
The archetypes supported Kepler's realist stance in two ways. They justified his method as truth-linked, and they accounted for the accessibility to the human mind of nature's underlying structure. In particular, they justified his view that fruitfulness and simplicity are theoretical virtues on the basis of which to adjudicate rival hypotheses. Because God's essence is simple, and this simplicity is reflected in the material by archetypal correspondence, a theory that does not unify complex phenomena is unlikely to match the archetypes. As God imprinted us with the archetypes, we can rule out hypotheses that “disturbed the mind,” and as the archetypes imply the unification of diverse phenomena, we have ample opportunity to test hypotheses for error. Thus, the “linking of syllogisms” implies that even if two theories are empirically equivalent, they may not be evidentially equivalent. Hence, despite making the same predictions, one theory can be better supported than the other.
Kepler's conception of the archetypal universe was also instrumental in his justification of incorporating physics into astronomy. Without this justification, his answer to the problem of observational equivalence would have been suspect for it involved the merging of traditionally distinct disciplines. Moreover, conceiving of physical objects as essentially geometrical justified the use of mathematical models in describing physical systems. The analogy between things mechanical and celestial furthered this end. The idea that some mathematical relations have more aesthetic appeal than others and hence are more likely to be found in the world justified certain mathematical models as physically plausible when no qualitative physical description was available. We see this in particular in Kepler's discussion of libration in the Astronomia and Epitome.
The archetypes also served to rule out hypotheses that could not be represented geometrically, and to establish the candidacy of those that could. Both functions were of key importance to the “war on Mars,” in which Kepler broke new ground in physics and astronomy. Once he had abandoned the traditional means of representing planetary motion, he could use the archetypes to determine the plausibility of other means. In addition, the aesthetic appeal of the ellipse, its unification of his distance