 Today Kepler is remembered in the history of sciences above all for his three planetary laws, which he produced in very specific contexts and at different times. While it is questionable whether he would have understood these scientific statements as “laws”—and it is even arguable that he used this term with a different meaning than we do today—it seems to be clear that all three laws (as a linguistic convention, we may continue to use the term) suppose some fundamentals of Kepler’s philosophy: (a) realism, (b) causality, (c) the geometric structure of the cosmos. Besides this, it should be remarked that the common denominator of all three laws is Kepler’s defense of the Copernican worldview, a cosmological system which he was not able to defend without reforming it radically. It is noteworthy that already at the very beginning of his career Kepler vehemently defended the reality of the Copernican worldview in a way that he characterized, taking over the terminology from the standard Aristotelian epistemology, as “a priori” (see above Section 3 above and Di Liscia 2009).