The reappearance of Galileo's original Letter to Benedetto Castelli.

This article describes an important manuscript discovered recently in the Royal Society archives, and presents evidence that it is the holograph of Galileo's  Letter to Benedetto Castelli of 21 December 1613. It was in this letter that Galileo first set out his ideas on the relation between science and religion, and defended Copernican astronomy from charges of being contrary to the Holy Scriptures. The text of the  Letter  has hitherto been known only through manuscript copies, namely the 12 used by Antonio Favaro in his critical edition of 1895. Despite his magisterial work, Favaro did not manage to locate Galileo's autograph and was forced to rely exclusively on copies. The  Letter to Castelli  preserved at the Royal Society is of remarkable interest. By comparison with the other extant manuscript copies by different hands, its wording seems to be theologically more daring and compromising. The discovery of this autograph is therefore one of the most important in Galilean studies in recent decades, shedding new light on his intricate relations with the Roman Catholic Church in these years.