Beginning with a long and extensively rewritten introduction surveying the predecessors of the Presocratics, the book traces the intellectual revolution initiated by Thales in the early sixth century B.C.E. to its culmination in the metaphysics of Parmenides and the complex physical theories of Anaxagoras and the Atomists in the fifth century. It is based on a selection of some six hundred texts, in Greek and a close English translation which in this edition is given more prominence. These provide the basis for a detailed critical study of the principal individual thinkers of the time.