Galileo Galilei's Letter to Benedetto Castelli (1613) was his first statement on the authority of scripture and the Catholic Church in matters of scientific enquiry. In a series of bold and innovative arguments, he undermined the claims for Biblical authority which the opponents of Copernicus used. The letter was the subject of the first complaint about Galileo to the Inquisition in 1615. Background In 1610 Galileo had published Sidereus Nuncius (The Starry Messenger), which made him famous across Europe. This work prompted many debates as to whether the Earth really was the centre of the universe. Galileo usually avoided referring to scripture in his arguments about the universe, while the Aristotelian scholars who opposed Copernicus cited the Bible in support of their views – for example Lodovico delle Colombe in his 1611 work Contra il Moto della Terra (Against the Motion of the Earth) explicitly challenged anyone defending Copernicus to answer the charge that he was going against what the Bible taught. This presented Galileo with a dilemma – if he did not respond, he effectively conceded that the biblical text confirmed the Aristotelian view despite the fact that the Church had no firm position on the Copernican question; on the other hand if he tried to engage in arguments based on scripture, he allowed himself to be drawn into a field where the Church regarded its authority as absolute. Indeed an earlier venture into this form of argument by Galileo had been stopped by the Church. In the manuscript for Letters on Sunspots (1613), he argued that "flaws" in the Sun demonstrated that the heavens were not immutable, as had previously been thought. A paragraph in which Galileo supported this by claiming that the scriptures supported the mutability of the heavens was removed by the Inquisition censors. On 14 December 1613 Galileo's friend and former pupil Benedetto Castelli wrote to him to say that at a recent dinner in Pisa with the Grand Duke Cosimo II de' Medici a conversation had taken place in which Cosimo Boscaglia, a professor of philosophy, argued that the motion of the Earth could not be true, as it was contrary to the Bible. Castelli had disagreed with him and maintained, as Galileo held, that the Earth's motion was possible. After the dinner, Castelli had been called back by the Dowager Duchess Christina of Tuscany to answer points she raised from scriptural arguments against