Indeed no more remarkable difference distinguishes the modern world from all that went before than its attitude toward change itself. The medieval world idealized changelessness. Its very astronomy was the apotheosis of the unalterable. The earth, a globe full of mutation and decay; around it eight transparent spheres carrying the heavenly bodies, each outer sphere moving more slowly than its inner neighbour, while the ninth, moving most slowly of all, moved all the rest; last of all, the empyrean, blessed with changeless, motionless perfection, the abode of God--such was the Ptolemaic astronomy as Dante knew it. This idealization of changelessness was the common property of all that by gone world. The Holy Roman Empire was the endeavour to perpetuate a changeless idea of political theory and organization; the Holy Catholic Church was the endeavour to perpetuate a changeless formulation of religious dogma and hierarchy; the Summa of St. Thomas Aquinas was the endeavour to settle forever changeless paths for the human mind to walk in. To that ancient world as a whole the perfect was the finished, and therefore it was immutable. How different our modern attitude toward change has come to be! We believe in change, rely on it, hope for it, rejoice in it, are determined to achieve it and control it. Nowhere is this more evident than in our thought of the meaning of knowledge. In the medieval age knowledge was spun as a spider spins his web. Thinking simply made evident what already was involved in an accepted proposition. A premise was drawn out into its filaments and then woven into a fabric of new form but of the same old material. Knowledge did not start from actual things; it did not intend to change actual things; and the shelves of the libraries groan with the burden of that endless and largely futile cogitation. Then the new knowledge began from the observation of things as they really are and from the use of that observation for the purposes of human life. Once a lad, seventeen years old, went into the cathedral at Pisa to worship. Soon he forgot the service and watched a chandelier, swinging from the lofty roof. He wondered whether, no matter how changeable the length of its arc, its oscillations always consumed the same time and, because he had no other means, he timed its motion by the beating of his pulse. That was one time when a boy went to church and did well to forget the service. He soon began to wonder whether he could not make a pendulum which, swinging like the chandeliers, would do useful business for men. He soon began to discover, in what he had seen that day, new light on the laws of planetary motion. That was one of the turning points in human history--the boy was Galileo. The consequences of this new method are all around us now. The test of knowledge in modern life is capacity to cause change. If a man really knows electricity he can cause change; he can illumine cities and drive cars.