It was natural that the first fruits of the new science of the Renaissance should have been in astronomy and mechanics. The accumulated observations of the astrologers had vastly increased the complexity of the Ptolemaic system, and the discrepancies between the observed and calculated positions of the planets had become glaringly evident. When the great treatise of Copernicus was finally published as he lay on his death-bed, it is altogether probable that no one suspected that it marked the beginning of a new philosophy. He had merely proved that the calculations of astronomers were greatly simplified by assuming that the sun, instead of the earth, was the fixed centre of the solar system, and that the earth and other planets revolved about it in circular orbits. It must remain a mooted question whether Copernicus believed that his discovery was only a mathematical device; his book states explicitly that he, as a Catholic, still subscribed to the belief that the earth was actually the fixed centre of the universe as the Church and the Aristotelians both taught to be a necessary article of faith. At all events, it was not until Galileo some sixty years later invented the telescope and turned it on the heavens, that men saw the significance of the discovery. The eye of the telescope penetrated the depths of the solar system. It proved that the planets were not pure celestial matter but were mere masses like the earth. Their brilliance was not a divine fire but ordinary sunlight reflected from their dull surfaces; they, like the earth, were inanimate bodies revolving about the sun and the Copernican system became a fact instead of a mathematic device. It is not extravagant to assert that, with the acceptance of Copernican astronomy, the whole mediæval conception of nature gave place to a reliance on experimental evidence. The work of Galileo in founding the science of mechanics was fatal to the mysticism of the contemporary alchemists. Instead of the elements with their natural places, their likes and dislikes, their hierarchy of nobility, and their subserviency to planetary influences, he laid down the universal principle that all natural actions were due to mechanical forces whose only function was to alter the motions of bodies and whose amount was measurable in mathematical symbols. His significant work for us was an uncompromising war waged against the scientific dogmatism of the Aristotelians on the clear-cut issue that knowledge of the objective world could be obtained only by experimental evidence, and not by subjective preconceptions. Understood rightly, he had reinstated the Platonic dualism of two worlds, one of matter, and the other of the mind or spirit.