Galileo against Cardinal Bellarmine: a Defence of the Astronomical-Philosophical Program

Abstract

About 20 years ago Cardinal Poupard and Pope John Paul II re-actualized Duhem’s claim regarding Galileo being a good theologian and a bad epistemologist, who should accept Cardinal Bellarmine’s demand to speak – as in his opinion Copernicus did – about how the Earth moves and the Sun stands still only suppositionally. In this article, which is limited only to the epistemological aspect of the question, I first present the historical and epistemological context of the issue and then analyze Galileo’s answer to the Cardinal’s demand as developed in Galileo’s letter to Dini (23 March 1615) and in the so-called Considerations on the Copernican Opinion (1615). In this letter Galileo very convincingly argued, firstly, that Copernicus himself was an astronomical “realist” and secondly that even Ptolemaic astronomy was itself based on philosophical principles. This leads to the conclusion that the astronomical-philosophical program is the only plausible and possible solution. In his Considerations Galileo also indicated the criteria according to which one has to choose in the dilemma between the two contradictory systems, of which one is necessarily true and other necessarily false. In these texts, Galileo developed an epistemologically sound defence of the “realistic”, astronomical–philosophical program, which was first articulated in its heliocentric version by Copernicus and then accepted and defended by some other “real Copernicans” (for example Kepler) and in its geocentric version by the leading Jesuit astronomer Christopher Clavius.