However, it appears that Galileo was not convinced that his student had sufficiently clarified the issue. On December 21, he sent a letter to Castelli, outlining in detail his personal opinion on the unsuitability of the Bible as a source of arguments to disprove science. "I believe that the authority of Holy Scripture has the sole purpose of convincing people of truths and ideas that are necessary for our salvation and which, due to our inability to understand them with our reason, could not be convincingly proven through any other science," he wrote. Without wasting time, of course, he continued, with the sarcastic tone characteristic of most of his works, that he found it difficult to believe that "the same God, who endowed us with senses, reason, and intelligence, wished us to abandon their use." In other words, Galileo claimed that when the Scriptures seem to conflict with what is empirically observed and proven about nature, they should be interpreted differently. He pointed out that this is "particularly true regarding topics for which the Bible contains few passages and only some conclusions, just as is the case with astronomy, which is given so little space in the Bible that not all planets are even listed." Although this argument was not entirely new—since as early as the 5th century, the theologian Saint Augustine had written that the authors of sacred texts did not intend to teach scientific matters, "since such knowledge did not contribute at all to the salvation" of the soul—Galileo's bold statements ultimately led him into conflict with the Catholic Church.