chapter 6 A priori and a posteriori: Two Approaches to Heliocentrism One could consider the earliest reception of De revolutionibus to be concluded with the publication of Mysterium cosmographicum, an ambitious attempt to reaffirm the reality of heliocentrism which signaled its author, the young Kepler, as one of the most promising and original mathematical and philo- sophical minds of his time. He did not embrace the geo-heliocentric “third way” of Ursus and Brahe, and would later reject the infinitist viewpoint of Bruno and his followers. Rather, he reassessed the Copernican system from a completely new perspective. After a period of intense astronomical observa- tions (the approach supported by Landgrave Wilhelm IV and Brahe as well as by Mästlin and Magini), Kepler claimed that it was possible to grasp the design of the heavens from an a priori perspective. His intention was, in fact, to unveil the archetypal reasons for the planetary order rooted in Divine Providence. He called this hidden astronomical truth, in Latin, the mysterium cosmographi- cum, i.e. the cosmic secret. An important aspect of his speculations was the project of unifying mathematical and physical astronomy, which he would especially develop in Astronomia nova (1609) and in Harmonice mundi (1619). On the other hand, Galileo’s telescopic discoveries, first communicated in Sidereus nuncius (1610), strengthened the heliocentric cause by bringing new data that were not reconcilable with either Ptolemaic geocentrism or with the Aristotelian principle that the heavens are unalterable. The Copernican alliance between Kepler and Galileo was in many respects a historical contingency, since they came to support the heliocentric system for different reasons and starting from very different conceptions of science. Among other historians, Ludovico Geymonat, in his Galileo Galilei (1957), dis- tinguished the “engineering” and experimental use of mathematics by Galileo from the “numerologies” of Kepler. From a different perspective, the art his- torian Erwin Panofsky, in Galileo as a Critic of the Arts (1954), contrasted the “progressive empiricism” of the former and the “idealistic conservatism” of the latter. Koyré, who was significantly influenced by Panofsky, considered Kepler’s concept of harmony pre-modern, since it was intimately linked to the idea of an ordered and finite cosmos, which was to be overthrown, according to his narrative, by the “modern scientific outlook.” More recently, Massimo Bucciantini, in Galileo e Keplero, traced the parallel developments of the works © koninklijke brill nv, leiden, ���4 | doi ��.��63/9789004�54503_��8 A Priori and a Posteriori 235 by these two authors and argued for their complementarities, emphasizing the mutual esteem of the authors. Yet, Bucciantini also stressed their differ- ent philosophical approaches: “The Galilean project to found a Copernican science of motion was, from the beginning, in opposition and concurrence to Kepler’s new celestial dynamics. [. .] Hence derive two different manners to be modern, that is to say, to be philosophers and scientists who supported Copernicus’s views [. .].”1 In the following I will highlight the epistemological premises of these two approaches, mainly focusing on Mysterium and Sidereus nuncius. This treat- ment, especially if compared with chapter two, will make clear the differ- ent philosophical framework that emerged in the late sixteenth and early seventeenth centuries and paved the way for the natural investigations and visionary worldviews of the Baroque. As Robert Westman pointed out in The Copernican Question, the passage from Kepler’s and Galileo’s generation to that of Descartes and his successors was marked by a shift from a discussion on planetary models and their physical justification to natural philosophy, that is, philosophical attempts at embedding astronomical issues in mechanical conceptions of nature and at deriving planetary models from the most general principles of nature.2 1 Mästlin’s a posteriori Astronomy I would start from the University of Tübingen, around which, for a long time in the late sixteenth century, the work of two convinced champions of Copernicus’s heliocentrism gravitated: Michael Mästlin and his pupil Kepler. It has already been remarked in studies on Renaissance astronomy that Mästlin’s publications show a sharp divergence between his research and his teaching activities with regard to the acceptance of the heliocentric cosmol- ogy. In the works directed to a learned readership of specialists, he praised the hypotheses of Copernicus, while in his books destined for teaching he main- tained a strictly Ptolemaic approach. Considering, for instance, the Epitome astronomiae (Summary of Astronomy), first published in 1582 (but with many reprints and several new editions), one is struck by the elementary nature of the exposition.3 However, this was a text with a pedagogical intent, based on the model of Sacrobosco. The Earth was presented as the center of the world 1 Bucciantini, Galileo e Keplero, 336. Cf. Field, “Cosmology.” 2 See also, as a standard reference, Dijksterhuis, Mechanization. 3 See Methuen, “Maestlin’s Teaching.”.