the motion of the Earth. Castelli had responded and Boscaglia had remained silent. Castelli wished to alert Galileo to this exchange, and advised Galileo that their mutual friend Niccolò Arrighetti would come to Florence and explain matters further. This Arrighetti did. Galileo felt that it was important for him to set out an argument to show how scripture could not be used as the basis for scientific enquiry. He did so with great speed, replying with a letter to Castelli in less than a week, on 21 December 1613. His Letter to Benedetto Castelli was not published, but was circulated widely in manuscript form. As the debate about its arguments continued, Galileo thought it advisable to review and expand the arguments he had set out. This was the basis of his subsequent Letter to the Grand Duchess Christina, which expanded the eight pages of his letter to Castelli to forty pages. Key arguments In his letter to Benedetto Castelli, Galileo argues that using the Bible as evidence against the Copernican system involves three key errors. Firstly, claiming that the Bible shows the Earth to be static and concluding that the Earth therefore does not move is arguing from a false premise; whether the Earth moves or not is a thing which must be demonstrated (or not) through scientific enquiry. Secondly, the Bible is not even a source of authority on this kind of question, but only on matters of faith - thus if the Bible happens to say something about a natural phenomenon, this is not sufficient for us to say that it is so. Thirdly, he shows by deft argument that it is open to question whether the Bible, as his opponents claimed, even contradicted Copernicus' model of the universe. Indeed, Galileo argues, a key passage in the Bible which was held by his opponents to support the view that the Sun moves round the Earth supports his own views much better. In the Bible Joshua 10:12 is an account of how God commanded the Sun to stand still so that Joshua could defeat his enemies. According to those opposed to Copernicus, this showed clearly that the Sun (and not the Earth) moved. Galileo argued that this passage could not be used to support the traditional Earth-centred view of the universe at all. If we assume the universe to be as it was described by Claudius Ptolemy, the Sun's