Whenever naturalism and scepticism have thus stood opposed, the result has been their transformation or absorption into a new philosophy, combining the systematic formalism of the one with the introspective idealism of the other. In Greece such a revolution had already been effected once before by Plato; and a restoration of his system seemed the most obvious solution that could offer itself on the present occasion. Such was, in fact, the solution eventually adopted; what we have to explain is why its adoption was delayed so long. For this various reasons may be offered. To begin with, the speculative languor of the age was unfavourable to the rise of a new school. Greece was almost depopulated by the demands of foreign service; and at Alexandria, where a new centre of Hellenism had been created, its best energies were absorbed by the cultivation of positive science. It was, no doubt, in great part owing to the dearth of ability that ideas which, at an earlier period, would have been immediately taken up and developed, were allowed to remain stationary for a hundred years—the interval separating a Carneades from an Arcesilaus. The regular organisation of philosophical teaching was another hindrance to progress. A certain amount of property was annexed to the headships of the different schools, and served as an endowment, not of research but of contented acquiescence in the received traditions. Moreover, the jealousy with which the professors of rival doctrines would naturally regard one another, was likely to prevent their mutual approximation from going beyond certain not very close limits, and might even lead to a still severer definition of the characteristic tenets which still kept them apart. Another and deeper disturbing force lay in the dissensions which, at a very early stage of its development, had split the spiritualistic philosophy into two opposing tendencies respectively represented by Plato and Aristotle. Any thinker who wandered away from the principles either of Stoicism or of Scepticism was more likely to find himself bewildered by the conflicting claims of these two illustrious masters, than to discern the common ground on which they stood, or to bring them within the grasp of a single reconciling system. Finally, an enormous perturbation in the normal course of speculation was produced by the entrance of Rome on the philosophical scene. But before estimating the influence of this new force, we must follow events to the point at which it first becomes of calculable importance.