Abstract
In his 1615 letter to the Grand Duchess Christina of Lorraine, Galileo argues for a “principle of limitation”: the authority of Scripture should not be invoked in scientific matters. In doing so, he claims to be following the example of St Augustine. But Augustine’s position would be better described as a “principle of differing purpose”: although the Scriptures were not written in order to reveal scientific truths, such matters may still be covered by biblical authority. The Roman Catholic Church has rejected Galileo’s principle, opting rather for Augustine’s, leaving open the possibility of future conflicts between scientists and Church authority.