§ 66. _Epistemology is the theory of the possibility of knowledge_, and issues from criticism and scepticism. If we revert again to the history of Greek philosophy, we find a first period of enterprising speculation giving place to a second period of hesitancy and doubt. This phase of thought occurs simultaneously with the brilliantly humanistic age of Pericles, and it is undoubtedly true that energy is withdrawn from speculation largely for the sake of expending it in the more lively and engaging pursuits of politics and art. But there are patent reasons within the sphere of philosophy itself for entailment of activity and taking of stock. For three centuries men have taken their philosophical powers for granted, and used them without questioning them. Repeated attacks upon the problem of reality have resulted in no consensus of opinion, but only in a disagreement among the wise men themselves. A great variety of mere theories has been substituted for the old unanimity of religious tradition and practical life. It is natural under these circumstances to infer that in philosophy man has overreached himself. He would more profitably busy himself with affairs that belong to his own sphere, and find a basis for life in his immediate relations with his fellows. The sophists, learned in tradition, and skilled in disputation, but for the most part entirely lacking in originality, are the new prophets. As teachers of rhetoric and morals, they represent the practical and secular spirit of their age; while in their avoidance of speculation, and their critical justification of that course, they express its sceptical philosophy. [Sidenote: Scepticism, Dogmatism, and Agnosticism.]