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.