Galileo then demonstrates the vast difference between doctrinal and exact sciences, and says that in the latter opinions cannot be changed to order. Supported by the authority of St. Augustine, he maintains that opinions on natural science which have been proved to coincide with actual facts cannot be set aside by passages of Scripture, but these must be explained so as not to contradict the indisputable results of observation. Those, therefore, who desire to condemn an opinion in physics must first show that it is incorrect. But it must be made the subject of close investigation, and then a different result will often be obtained from the one desired. Many learned men who intended to refute the Copernican theory have been changed, by examination, from opponents to enthusiastic defenders of it. In order to banish it from the world, as many desired, it would not be enough to shut the mouth of any one individual, it would be necessary to prohibit not only the writings of Copernicus and his followers, but astronomy altogether. But to suppress his work now, when new discoveries are daily confirming his theory, after it has been quietly submitted to for so many years, appears to Galileo like opposition to truth itself; and to permit the book and condemn the doctrine would be still more pernicious to the souls of men, for it would allow them the opportunity of convincing themselves of the truth of an opinion which it was a sin to believe. To forbid astronomy altogether would be like rejecting hundreds of passages of Scripture which teach us how the glory of God is revealed in all His works, which are best to be studied in the open book of nature. Galileo then applies these general principles to the Copernican theory. According to many, it ought to be pronounced erroneous because it is opposed to the apparent meaning of many passages in the Bible, while the opposite opinion is to be believed _de fide_. He sharply defines two kinds of scientific questions: those on which all man’s researches can only lead to probability and conjecture, as for instance, whether the stars are inhabited or not; and those on which, by experience, observation, and inevitable deduction, we either have attained certainty or may safely reckon on doing so,—as whether the earth or the heavens move. In the first case, Galileo is decidedly of opinion that it behoves us to be guided by the literal sense of Scripture; in the second, he repeats what he has said before, that two truths can never contradict each other. The Bible speaks of the sun as moving and of the earth as standing still to accommodate itself to the understanding of the people, and not to confuse them, otherwise they might refuse to believe the dogmas which are absolutely _de fide_. For the same reason the fathers have spoken about things not appertaining to salvation, more in accordance with usage than actual facts, and he confirms this by quotations from St. Jerome and St. Thomas.