Letter to Christine of Lorena, Galileo Galilei, Malatestiana Library, Cesena, Italy, Europe

The letter addressed to Christine of Lorraine Grand Duchess of Tuscany was begun by Galilei in February 1615 and completed in the summer of the same year. The months that took place in the composition are explained by the large amount of writing, which can be defined as a brief theoretical treatise rather than a letter of clarification. It was a crucial moment for Galilei's scientific activity: since his Sidereus Nuncius had been around Europe in 1610, Copernicanism was no longer a simple hypothesis or a means of simplifying astronomical calculations, but a thesis that observations allow us to predict that it would soon be possible to demonstrate, thus undermining the ancient domain of geocentrism. Breaking down the venerable Aristotelian-Ptolemaic cosmology meant upsetting the anthropocentric image of the universe that had been consolidated through the medieval centuries. Thus the problem of the veracity of the Copernican theory quickly shifted from a terrain of philosophy alone to the religious and theological field, a shift facilitated also by the post-Tridentine cultural climate which, through instruments such as the index of prohibited books, favored ecclesiastical supervision of intellectual production. The letter to Madama Cristina di Lorena represents Galilei's most mature theoretical attempt in this effort to clarify and restructure the relationship between knowledge, an attempt that aimed at the de-responsibility of the Bible from its use as a source of authority in scientific research. In the letter Galilei used four fundamental theoretical principles: the principle of inerranza, the principle of the only source of truths, the principle of limitation, the principle of prudence. Galilei used these four theoretical principles to found the key concept that was to guarantee the autonomy of research of natural philosophy from the authority of the sacred text: it is never methodologically correct to use biblical verses as experimental proofs. This criterion was already evident in the Augustinian De Genesi ad Litteram and it is no coincidence that Galilei mentions it abundantly. For Galileo the Bible had to be scientifically de-empowered because between nature and Scripture, although both coming from the same divine Word, there is an irreducible inhomogeneity both of language and of purposes. The book of nature is univocal, for those who possess the mathematical-geometric knowledge it speaks clearly without metaphors or allusions. The Holy Scriptures, on the other hand, are a complex text, multi-layered and in need of a careful work of interpretation: behind the often harsh language used to adapt to the understanding of the common man the Bible hides for the learned treasures of wisdom.