It is significant that during Galileo’s lifetime, so swift was the movement, Descartes saw the trend of the new science and its inevitable effect on philosophy and religion. In his _Système du Monde_, he pictured a universe of matter and motion and nothing else,--a machine. From matter he tried to strip every sensible attribute except extension, or its mere geometrical position and extent. All phenomena became for him merely phases of motion. With the courage of his conviction, he even tried to imagine plants and animals to be mechanically acting automata. One thing only he could not include in this mechanism,--and that was _thought and consciousness_. The cosmogony of Descartes has long since crumbled to dust, but the gap he made between the subjective and objective worlds has never been closed in spite of incessant later attempts. And these early creators of science, as something distinct from humanism, were quite conscious that they were engaged in a revolution which could end only with the overthrow of the dogmatic science of the Middle Ages, sheltered by the authority of Aristotle and the Bible. Galileo, Descartes, Pascal, and Bacon all declared explicitly that the old order must pass. From their day to ours, we have more and more regarded the universe as a machine, a combination of inert matter and moving forces, acting not as we may desire, but according to invariable laws which we have personified as Nature. Man was left, by the physicists, as a unique outsider who could interpret this machine in terms of his sensations and mind, but could neither alter nor avoid its fateful operation. Only one step further was needed to identify all science with mechanics and to compress the scientific method within the limits imposed by mechanical laws. This fundamental principle was Newton’s discovery of the universal attraction of matter. According to this law, a single kind of force, depending only on the amount of matter and the distance between bodies, operated to hold the stars in their paths and to cause all the chemical and physical activities of atoms. With its discovery, the mechanistic theory was complete and dominant, and science had, in principle, gone as far as it can ever go. But, what is even more important, Newton grasped in his early years the fact that the scientific method is limited to the experimental investigation of objective phenomena, those which can be classified, measured geometrically, and formulated in laws which predict future events. He held with absolute consistency and restraint that what is once accomplished by this method is permanent. The experimental laws of gravitation, of the pressure of gases, and all others of like nature, are true within the limits of accuracy of our observation and measurement. If these improve, such laws do not fail, their mathematical expression is merely made more precise. They belong to the permanent acquisitions of the mind. III