Kepler's epistemology was profoundly concerned with astronomical practice, with methodological issues, and with the results of their application to the serious problems of late-renaissance astronomy. Kepler not only was able to ask questions in a way that no one had before asked, but he was able to provide answers to those questions that even now are worthy of continued study. From this perspective, Kepler was less the last great cosmologist of the classical tradition that includes Ptolemy and Copernicus; more was he the first cosmologist of the modern tradition.
In the middle of the sixteenth century, fourteen centuries after the death of Ptolemy (c. A.D. 100-170), his Almagest still dominated all of astronomy. With the renaissance of interest in the works of the ancients, the relevance of the Ptolemy's great work had not only not diminished, but was on the increase (van Helden, 42). But by the end of the sixteenth century and the beginning of the seventeenth the Ptolemaic system was facing serious challenges. The subsequent revolutionary transformation from the geocentric to the heliocentric worldview has been almost universally attributed to the works of Nicholas Copernicus (1473-1543). Copernican astronomy is best known for the radical proposition that the Earth is in motion about the Sun rather than vice versa. Copernican heliocentrism emerged from a profound dissatisfaction with the fundamental principles upon which Ptolemy had based his system of the heavens. In particular, the Ptolemaic assumption of the punctum equans Copernicus viewed as contrary to the admissible explanations for the true motions of the planets. The punctum equans, or equant point, is an eccentric point about which the planet is supposed to subtend a uniform angular speed. Historically, published in 1956, Johannes Kepler's first and evidently most influencial astronomical work Mysterium Cosmographicum was the first major treatise after Copernicus' De revolutionibus to argue for heliocentrism (Gingerich, 347). Although Kepler has been defending the theory of Copernicus from the very beginning, with Mysterium Cosmographicum scientist provided a new kind of theory of the planets. His theory of the planets has been classified as new because it was the product of the first explicit and thorough attempt to consistently unify the epistemological structures of the hitherto divergent sciences of astronomy and physics (Koyre, 119). Kepler's objective in Mysterium Cosmographicum was nothing less than the development of a theory of the absolute structure of the world system. Kepler was certainly not the first to attempt to provide a general cosmographic account of the planets, that is, an account that seeks to explain the proportions of the universe as a whole. Both Ptolemy's and Copernicus' respective theories each maintain not only an ordering of the planetary orbs, but contain estimates of the ratios of their dimensions. But in the theory of Ptolemy these estimates are empirically underdetermined because of the independence of each planetary hypothesis, and in the theory of Copernicus, the estimates are strictly a posteriori consequences of the heliocentric hypothesis and, because of their reliance on Ptolemaic observations and mathematical constructions, no better corroborated