The New Astronomy.—The general criticisms of Cusanus were elaborated by Copernicus. The senses cannot inform us (when any motion takes place) what it is that moves. It may be the thing perceived that moves, or the percipient—or both. And it would be possible to account for the movements of celestial bodies by the supposition that it is the earth that moves, and not they. Copernicus' whole work consisted in the mathematical demonstration that this hypothesis could account for the phenomena as we observe them. In fact, when these demonstrations were eventually published (it was only on his death-bed that Copernicus received a copy of his book—and he had already lost consciousness) they were introduced by a discreet preface, which intimated that the whole thing might safely be regarded as a jeu d'esprit on the part of an eccentric mathematician. And this editorial caveto, though written by another hand, preserved the Copernican theories from the notoriety that might otherwise have attended, and afterwards did attend, them.
Copernican conceptions were semi-traditional. The sun displaces the earth as the central point of the universe: around it revolve the planets—including the earth; and, at an immeasurable distance, is the immovable heaven of the fixed stars. Copernicus left it an open question whether or no the universe was infinite. It remained for his successor, the greatest of the Renaissance thinkers, Giordano Bruno (1548-1600) to declare it to be limitless, and to contain an infinity of worlds like our own. The fixed stars became, for him, suns surrounded by planets. The traditional distinction between the celestial and sublunary spheres had vanished. The bewilderment and indignation excited by these ideas, revolting to the conscience of his time, cost their author his life.
Galileo.—The criticism of the old world-conceptions was, however, to be based on yet more sure ground by one who relied, not on general considerations, but on observation and experiment. Galileo (1564-1642) studied philosophy, physics, and mathematics at Pisa; and as professor expounded the old astronomy long after he had ceased to regard it as adequate. Not until 1610, after he had constructed a telescope and observed the satellites of Jupiter, did he openly confess his adherence to the system of Copernicus. The observation of sun-spots and the phases of Venus confirmed his opinion.
Aristotelian astronomers declined to witness these phenomena through his telescope, and perhaps Galileo was right in observing with a sigh that were the stars themselves to descend from heaven to bear him witness his critics would remain obdurate.[4]
It was not until 1632 that a complete exposition of the conflict between the two world-systems was produced by Galileo. It took the form of a dialogue between three speakers—conservative, mediating, and extreme. The views of the author, however, were not sufficiently concealed, the book was prohibited, and Galileo summoned to Rome, and upon threat of torture, subdued into a recantation and a promise not to offend in the future. That Galileo perjured himself is not open to doubt, nor did he change his convictions. A subsequent work, surreptitiously printed in Holland, contained the same heresies expressed with less reserve.
The New Physics.—It might be said, then, that the fabric of the universe had been reconstructed by the thinkers whose explorations we have hitherto followed. This achievement, however, though sufficiently startling in itself, was not the only, and perhaps not the most important, of their performances. The question still awaited solution: By what forces and laws is the new world-system maintained in activity?
The traditional reply had been that the universe was kept in motion by the operation of the Deity. While the truth of this reply was not questioned by the advocates of the "new" science, it did not seem to them to dispel the obscurity surrounding certain points about which they required information. It was Galileo who observed that the appeal to the divine will explains nothing just because it explains everything. It takes the inquirer back too far—behind those details of method which arouse his speculative interest.
This desire to understand those methods of operation which natural objects appear to follow, led philosophers to enunciate certain "laws" about them. These served as "explanations" of particular classes of phenomena. It was the phenomena of motion that especially attracted their attention; and many ingenious experiments were performed by Galileo, in particular, which led him to conclusions which then seemed paradoxical, but now serve as axioms of physical science; for "the laws of motion contain the key to all scientific knowledge of material nature." When Galileo, after careful experiment, established the proposition that a body can neither change its motion of itself, nor pass from motion to rest, the fundamental "law of inertia"—of such incalculable importance to the development of modern physics—had been established.
An Automatic Universe.—A proposition of this kind may not at first seem to involve important philosophical or theological consequences. But we only have to consider that it provided a natural explanation of the continued and untiring motion of the heavenly bodies. It did not, it is true, explain how that motion arose; but the motion being "given," it had now been shown how it would, in the absence of obstructions, be perpetual. In fact, speculations of this kind opened up the way to the mechanical explanation of nature, a theory which had been already speculatively held by Leonardo da Vinci, who is already convinced that "necessity is the eternal bond, the eternal rule of Nature."
Science and Mathematics.—It was not only, however, the spectacle of a system running automatically that suggested to observers a mechanical theory to explain it. There was also the fact that phenomena were observed to occur in accordance with certain simple mathematical laws. Galileo's experiments with falling bodies led him to foreshadow principles which were afterwards elaborated and fully demonstrated by Newton, who may be said to have been the first to construct a mechanical universe. The principle had already been formulated by a contemporary of Galileo—Johannes Kepler—in the axiom ubi materia, ibi geometria.