The origins both of modern science and modern philosophy lie in Greek civilization of the 5th and 6th centuries B.C. It was then that a series of thinkers, usually known as the Presocratic philosophers, created ways of looking at the world that were fundamentally new. In the middle of social and political changes, and exposed to intellectual influences from the Near East as well as to traditional Greek ideas, the first Presocratics, Thales and Anaximander of Miletus, had a vision of a universe governed by absolute and impartial law. In terms of this idea they and their successors tried to account for the observed structure of the physical world. An increasing awareness of the philosophical problems invloved in this attempt led to the striking and enigmatic pronouncements of Heraclitus, and to the struggle to escape from self-contradiction in which Parmenides created the first philosophical arguments and the beginnings of conceptual analysis. By 450 B.C. the thought of these men was having repercussions in wider areas of Greek culture, and was an important factor in the great outburst of intellectual energy in the sophistic age - the last half of the 5th century. This book presents a picture of these developments, using, wherever possible, translations of the surviving fragments of the Presocratics as a foundation for the discussion.