To write a history of Greek philosophy is to describe the formative period of our own thought, the making of the framework which supported it until at least the latter part of the nineteenth century. The discoveries about the nature of matter (if that term may still be used), the size and character of the Universe, and the human psyche which scientists have been making during the last hundred years are indeed so revolutionary that they may result in a radical reshaping of our fundamental outlook. Apart, however, from the fact that they are still in such a state of rapid transition that it is difficult to see what this new framework of thought will be, the conservatism of ordinary human minds ensures that much in the older outlook will continue to colour our general presuppositions for a long time to come. Even the modem natural philosopher who studies the records of the earliest European thinkers may find that he has more in common with them than he expected. It is this fundamental and dateless character of much Greek thought which makes it worth while to attempt a fresh presentation of it for a contemporary reader.

There is another side to the coin. With the Greeks we stand at the beginning of rational thought in Europe. It follows that we shall not only be concerned with reasoned explanation or scientific observation, but shall be watching the emergence of these activities from the mists of a pre-scientific age. This emergence is not sudden, but slow and gradual. I shall try indeed to justify the traditional claim of Thales to be regarded as the first European philosopher; but I shall not intend by that to assert that at one bound the line was crossed between pre-rational, mythical or anthropomorphic conceptions and a purely rational and scientific outlook. No such clearly-marked line existed, or exists today. Besides appreciating what is of permanent value in Greek thought, we may also learn from observing how much latent mythology it continued to shelter within what appear to be a roof and walls of solid reason.