Philosophy, as Plato tells us, begins with wonder; and, staring open-eyed at the starry heavens on the plains of Mesopotamia, man took a first step in the careful observation of Nature, which carried him a long way in his career. But he was very slow to learn the second step--how to interrogate Nature, to search out her secrets, as Harvey puts it, by way of experiment. The Chaldeans, who invented gnomons, and predicted eclipses, made a good beginning. The Greeks did not get much beyond trained observation, though Pythagoras made one fundamental experiment when he determined the dependence of the pitch of sound on the length of the vibrating cord. So far did unaided observation and brilliant generalization carry Greek thinkers, that there is scarcely a modern discovery which by anticipation cannot be found in their writings. Indeed one is staggered at their grasp of great principles. Man can do a great deal by observation and thinking, but with them alone he cannot unravel the mysteries of Nature. Had it been possible the Greeks would have done it; and could Plato and Aristotle have grasped the value of experiment in the progress of human knowledge, the course of European history might have been very different. This organon was absent, and even in the art of medicine Hippocrates with all his genius did not get beyond highly trained observation, and a conception of disease as a process of Nature. The great Pergamite, Galen, did indeed realize that the bare fact was only preliminary to the scientific study of disease by experiment, and to the collecting of data, from which principles and laws could be derived. On the dark horizon of the ancient world shone the brightness of the Grecian dawn so clearly that the emancipated mind had an open way. Then something happened--how, who can tell? The light failed or flickered almost to extinction: Greece died into a mediaevalism that for centuries enthralled man in chains, the weary length of which still hampers his progress. The revival of learning awakened at first a suspicion and then a conviction that salvation lay in a return to the old Greek fathers who had set man's feet in the right path, and so it came about that in the study of chemistry, and in the inventions of Copernicus, Kepler and Galileo, modern science took its origin. The growth of the experimental method changed the outlook of mankind, and led directly to the development of the physical and biological sciences by which the modern world has been transformed.