From Burnet's Introduction

I. The Cosmological Character of Early Greek Philosophy
IT was not till the traditional view of the world and the customary rules of life had broken down, that the Greeks began to feel the needs which philosophies of nature and of conduct seek to satisfy. Nor were those needs felt all at once. The ancestral maxims of conduct were not seriously questioned till the old view of nature had passed away; and, for this reason, the earliest philosophers busied themselves mainly with speculations about the world around them. In due season, Logic was called into being to meet a fresh want. The pursuit of cosmological inquiry had brought to light a wide divergence between science and common sense, which was itself a problem that demanded solution, and moreover constrained philosophers to study the means of defending their paradoxes against the prejudices of the unscientific. Later still, the prevailing interest in logical matters raised the question of the origin and validity of knowledge; while, about the same time, the break-down of traditional morality gave rise to Ethics. The period which precedes the rise of Logic and Ethics has thus a distinctive character of its own, and may fitly be treated apart.¹

1. It will be observed that Demokritos falls outside the period thus defined. The common practice of treating this younger contemporary of Socrates along with the "Pre-Socratics" obscures the historical development altogether. Demokritos comes after Protagoras, and he has to face the problems of knowledge and conduct far more seriously than his predecessors had done (see Brochard, "Protagoras et Démocrite," Arch. ii. p. 368).