THE HISTORY OF SCIENCE is the history of systems of thought about the natural world. Though the most obvious characteristic of science in modern civilisation is the control it has given over the physical world, even while such practical control was being acquired, and certainly for long periods before it became possible, men were trying to bring nature within the grasp of their understanding. The inventions and practical achievements of applied science are of great interest to the historian and so are the effects of natural science on the layman's view of the world as seen in literature, art, philosophy and theology; of even greater interest is the internal development of scientific thought itself. The chief problems before the historian of science are, therefore: what questions about the natural world were men asking at any particular time? What answers were they able to give? And why did these answers cease to satisfy human curiosity. An obsolete system of scientific thought, which may appear very strange to us looking back from the 20th century, becomes intelligible when we understand the questions it was designed to answer. The questions make sense of the answers, and one system has given place to another not simply because new facts were discovered that falsified the old system, but more significantly because for some reason, sometimes the result of fresh observations, scientists began to rethink their whole position, to make new assumptions, to ask new questions, to look at long familiar evidence in a new way.
In ancient Greece men were concerned with trying to discover the intelligible essence underlying the world of change, and they pursued natural science more for understanding than for use. In their search for what was permanent in the changing world of observation, the Greeks hit upon