An Early Modern Treatise on Realism
THE APOLOGIA PRO TYCHONE CONTRA URSUM, Kepler's most epistemologically focused work, was written around 1600 but not published until 1858. Jardine speculates that had it been published in Kepler's lifetime, and had it received due attention in the interim, it would now “be a classic on a par with such seminal reflections on the nature of human inquiry as the Novum organum and the Discourse on Method” (Jardine 1984, 5).1 Instead, like many of Kepler's works, it has been studied only by a few. What is rather striking about the Apologia is that Kepler anticipated and provided sophisticated approaches to certain problems facing modern-day scientific realists, to wit, the problems of empirical equivalence, and the related problem that empirical success is not necessarily an indication of truth.2 Another respect in which the Apologia should be of interest to philosophers of science is that Kepler's grappling with philosophical issues engendered his novel approach to, and subsequent discoveries in, physical astronomy.
Although the Apologia reads as though it could have been written in the twentieth century, the roots of Kepler's realism are to be found in his Neoplatonic cosmology.3 My purpose here is to trace Kepler's response to the skeptics and to show how his response was motivated and justified by his cosmology.4 Of particular interest here is that whereas the focus in the Mysterium was primarily on formal and final causes of astronomical phenomena, in the Apologia we find much more attention paid to efficient causes.
The Apologia, though a philosophical treatise, was directed at a specific opponent, namely the imperial mathematician Nichol Baer (or Ursus), in defense of Tycho Brahe in his priority dispute over the invention of the Tychonic system.5 Kepler was obliged to write the Apologia in order to gain access to Brahe's remarkable observations. Kepler, embarrassed by the very idea of a priority dispute, compromised by writing a philosophical treatise on the historical and current status of astronomical hypotheses. Although Kepler did not directly address the charge of plagiarism, the Apologia did function as a defense against Ursus's claims that the geoheliocentric system could be found in the works of the ancients and that, in any