In his _Organon_ Aristotle makes it evident that the demand for a regulative art of scientific discourse was created by the eristic logic-chopping of those who were most deeply influenced by the Eleatic philosophy. Indeed, the case is quite parallel to the rise of the art of rhetoric. Aristotle regarded Empedocles as the originator of that art, as he referred the beginnings of dialectic to Zeno. But the formulation of both arts in well-rounded systems came much later. As men conducted lawsuits before the days of Tisias and Corax, so also were the essential principles of logic operative and effective in practice before Aristotle gave them their abstract formulation. While it is true, therefore, that the Pre-Socratics had no formal logic, it is equally true, and far more significant, that they either received from their predecessors or themselves developed the conceptions and the presuppositions on which the Aristotelian logic is founded. One of the objects of this study is to institute a search for some of these basic conceptions of Greek thought, almost all of which existed before the days of Socrates, and to consider their origin as well as their logical significance. The other aim here kept in view is to trace the course of thought in which the logical principles, latent in all attempts to construct and verify theories, came into play. It is impossible, no doubt, to discover a body of thought which does not ground itself upon presuppositions. They are the warp into which the woof of the system, itself too often consisting of frayed ends of other fabrics, is woven with the delight of a supposed creator. Rarely is the thinker so conscious of his own mental processes that he is aware of what he takes for granted. Ordinarily this retirement to an interior line takes place only when one has been driven back from the advanced position which could no longer be maintained. Emerson has somewhere said: "The foregoing generations beheld God and Nature face to face; we through their eyes. Why should not we also enjoy an original relation to the universe? Why should not we have a poetry and philosophy of insight and not of tradition, and a religion by revelation to us and not the history of theirs?" The difficulty lies precisely in our faith in immediate insight and revelation, which are themselves only short-cuts of induction, psychological short circuits, conducted by media we have disregarded. Only a fundamentally critical philosophy pushes its doubt to the limit of demanding the credentials of those conceptions which have come to be regarded as axiomatic.