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_. RESULTS.--The thinkers whose speculations have engaged us were indeed responsible for creating a revolution in ideas. For a finite universe whose centre was the earth, and which was kept in motion by the operation of the Deity, they had substituted the conception of illimitable space sown with innumerable systems like our own; and had created the beginning of a mechanical conception of nature.