The notion that knowledge is derived from a higher source than is practical activity, and possesses a higher and more spiritual worth, has a long history. The history so far as conscious statement is concerned takes us back to the conceptions of experience and of reason formulated by Plato and Aristotle. Much as these thinkers differed in many respects, they agreed in identifying experience with purely practical concerns; and hence with material interests as to its purpose and with the body as to its organ. Knowledge, on the other hand, existed for its own sake free from practical reference, and found its source and organ in a purely immaterial mind; it had to do with spiritual or ideal interests. Again, experience always involved lack, need, desire; it was never self-sufficing. Rational knowing on the other hand, was complete and comprehensive within itself. Hence the practical life was in a condition of perpetual flux, while intellectual knowledge concerned eternal truth. This sharp antithesis is connected with the fact that Athenian philosophy began as a criticism of custom and tradition as standards of knowledge and conduct. In a search for something to replace them, it hit upon reason as the only adequate guide of belief and activity. Since custom and tradition were identified with experience, it followed at once that reason was superior to experience. Moreover, experience, not content with its proper position of subordination, was the great foe to the acknowledgment of the authority of reason. Since custom and traditionary beliefs held men in bondage, the struggle of reason for its legitimate supremacy could be won only by showing the inherently unstable and inadequate nature of experience. The statement of Plato that philosophers should be kings may best be understood as a statement that rational intelligence and not habit, appetite, impulse, and emotion should regulate human affairs. The former secures unity, order, and law; the latter signify multiplicity and discord, irrational fluctuations from one estate to another.