A Brief History of Modern Philosophy. Harald Høffding

an ideal case, since absolutely empty space is unrealizable, but Galileo showed by the experiment of rolling a ball in a parchment groove that the length of time the ball continued in its course was in direct proportion to its own smoothness and the smoothness of the parchment. In this way he proved the principle of inertia. But Galileo likewise thought that circular motion, which he also regarded as simple and natural, as well as motion in a straight line, would be continuous if all external obstacles could be eliminated. In his investigations of the motion of falling bodies he likewise starts with the principle of simplicity, with a view to showing later that it is verified by observation and experiment. “If a stone, falling from a given position at considerable height, accelerates its speed, why should I not regard the acceleration as due to its simplest explanation? And there is no simpler explanation of acceleration than that of a continuously uniform increase.”—It follows further from the principle of inertia and the law of falling bodies that we must take account of the energy or the impetus of motion (energia, momento, impetu) present at each moment as well as the actual sensible motion.

      Galileo elaborated the modern theory of motion, which forms the basis of physics, in his Discorsi della nouve scienze (1638).—His Dialogo sopra i due massimi sistemi del mondo (1632) draws a comparison between the Ptolemaic and Copemican world-systems, without, as he thought, taking sides, but in such a way as to leave no doubt as to his real opinion. This brought on the catastrophe of his life. He had even previously (after the discovery of the moons of ​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 ​things; they are the primary and real attributes of things (primi e reali accidenti). Our disposition to regard taste, smell, color, heat, etc., as the absolute attributes of things, on the other hand, is due to sense-prejudice. We give these names to things when they furnish the occasion of certain sensations, but these sensations take place within our bodies. They do not inhere in things. They would vanish if the corpo sensitivo were to vanish.—This doctrine, which contains the principle of the mechanical conception of nature, acquired vast importance in the investigations into the theory of knowledge in the following period.