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.
Even the general agreement of the fathers in the interpretation of any passage of Scripture of scientific import should, in Galileo’s opinion, only confer authority on it when the question has been discussed by many fathers with knowledge of both sides. But this is not the case with the question of the double motion of the earth, for it had not come up at all at that time, and it could not occur to the holy fathers to dispute it, for the current opinion was in entire agreement with the literal meaning of the Bible. It was not enough to say that the fathers had all believed that the earth stood still, and that therefore it was to be held _de fide_, for it was very possible that they never investigated it, and only held it as generally current. If they had done so and found it deserving of condemnation, they would have said so, but it had never been discovered that they had. The writings of Diego di Zuñiga show, on the contrary, that when some theologians began to consider the Copernican theory, they did not find it erroneous or contrary to Scripture. Moreover, no argument could be drawn from an unanimous opinion of the fathers, for some of them spoke of the sun as stationary, others of the _primum mobile_.