Now it is the momentum of this conception of reality as a stable and complete system of absolute ideas, the development of which we have just roughly sketched, that is so important historically. Why this conception of reality, which apparently grew out of a particular historical situation, should have dominated philosophic theory for over two thousand years appears at first somewhat puzzling. Those who still hold and defend it will of course say that this survival is evidence of its validity. But, after all, our human world may be yet very young. It may be that "a thousand years are but as yesterday." At any rate philosophy has never been in a hurry to reconstruct conceptions which served their day and generation with such distinction as did the Platonic conception of reality. And this is true to the evolutionary instinct that experience has only its own products as material for further construction. On the other hand, the principle of evolution with equal force demands that only as _material_, not as final forms of experience, shall these products continue. It may be that philosophy has not yet taken the conception of evolution quite seriously. At all events it is certain that long after it has been found that, instead of being eternal and complete, the concept undergoes change, that it has simply the stability and wholeness demanded by a particular and concrete situation; after it has been discovered, in other words, that the stability and wholeness, instead of attaching to the content of an idea, are simply the functions of any content used as a purpose--after all this has been accepted in psychology, the conception of truth and reality which arose under an entirely different conception of the nature of thought still survives.