Abstract

In the period from 1596 to 1609 Kepler succeeded in 'physicalizing' astronomy in a manner that had never before been achieved. He proposed for the first time a truly heliocentric system---a cosmology more innovative in most respects than that of Copernicus. While Kepler's importance in the history of science is frequently acknowledged, his complex and sometimes obscure texts have been given comparatively little attention. This deficiency is reflected in the fact that scholarly opinion on Kepler's natural philosophy has historically been quite diverse and, until fairly recently, even contradictory. With a few notable exceptions, few philosophers of science have concerned themselves with the details of Kepler's arguments. ;This work will maintain that the new astronomy for which Kepler argued is predicated on and supported by the concurrent introduction, evident in all of the writings that will be discussed, of a new kind of scientific epistemology. The Keplerian writings, both theoretical and applied, show an innovative and sophisticated awareness of the problems associated with theory appraisal, hypothesis testing, and observational error. Kepler's scientific programme was a bold attempt to overcome not only geocentric astronomy and cosmology, but was also a refutation of scepticism in the mathematical sciences. ;The epistemological theory and the corresponding methodological practices that I will identify in Kepler's works place upon astronomical hypotheses precisely the constraints that are required to overcome, the kind of scepticism adopted by many of Kepler's contemporaries. In this manner Kepler's natural philosophy can be seen as an extended argument against the 'compromise of Geminus', that is, the classical distinction between the geometrical study of astronomy and the causal-explanatory study of physics. I suggest that Kepler's success in refuting this distinction is attributable largely to two criteria the satisfaction of which Kepler distinguishes as necessary for the truth of hypotheses, criteria whose practical application can be found in the evolution of Kepler's planetary theory. The first of these Kepler refers to as the principle of "linking of syllogisms", and the second as the principle that all astronomical hypotheses entail unique "peculiar consequences in the physical realm". Kepler describes these principles most explicitly in his Apologia pro Tychone contra Ursum of 1600; but I will argue that they underwrite central passages in his earlier Mysterium cosmographicum of 1596, and are practically applied in the argumentation of the later Astronomia nova of 1609. Accordingly, the thesis is divided into chapters corresponding to these works. With each chapter I place the arguments of Kepler's particular works into the context of his general astronomical endeavour, making particular reference to these principles