Jupiter and of sun-spots) expressed himself publicly as favoring the Copernican system. When the College of the Inquisition, therefore, in the year 1616, placed Copernicus’ book on the Index, he is said to have promised Cardinal Bellarmin that he would neither defend nor disseminate the Copernican theory. He denied that the Dialogo was a violation of this promise on the ground that he had expressed himself hypothetically. But the book was forbidden, and the old man of seventy was required—under threat of torture—to solemnly abjure “the false doctrine,” that the earth is not the center of the universe and that it moves. The Inquisition held him under suspicion for the rest of his life and he was forced to have his works published in foreign countries.
It has already been observed that the Copernican theory beautifully illustrates the unwisdom of accepting our ideas as the expression of reality without further question. Galileo emphasized this phase of the new theory very strongly; “Think of the earth as having vanished, and there will be neither sun-rise nor sun-set, no horizon even and no meridian, no day and no night!” Later on he expanded this idea so as to include the whole of physical nature. In the Dialogue he takes occasion to observe that he had never been able to understand the possibility of the transubstantiation of substances. When a body really acquires attributes which were previously lacking, it must be explained by such a rearrangement of its parts as would neither destroy nor originate anything. This clearly asserts the principle that qualitative changes can only be understood when referred to quantitative changes. Galileo had already stated this view even more strongly in one of his earlier works (Il saggiatore, 1632). Form, magnitude, motion and rest constitute all that can be said of
