Another feature of the medieval theory, pertinent to our inquiry, was an idea which Christianity took over from Greek and Roman thinkers. In the later period of Greek history, which began with the conquests of Alexander the Great, there had emerged the conception of the whole inhabited world as a unity and totality, the idea of the whole human race as one. We may conveniently call it the ecumenical idea--the principle of the ecumene or inhabited world, as opposed to the principle of the polis or city. Promoted by the vast extension of the geographical limits of the Greek world resulting from Alexander's conquests, and by his policy of breaking down the barriers between Greek and barbarian, the idea was reflected in the Stoic doctrine that all men are brothers, and that a man's true country is not his own particular city, but the ecumene. [Footnote: Plutarch long ago saw the connection between the policy of Alexander and the cosmopolitan teaching of Zeno. De Alexandri Magni virtute, i. Sec. 6.] It soon became familiar, popularised by the most popular of the later philosophies of Greece; and just as it had been implied in the imperial aspiration and polity of Alexander, so it was implied, still more clearly, in the imperial theory of Rome. The idea of the Roman Empire, its theoretical justification, might be described as the realisation of the unity of the world by the establishment of a common order, the unification of mankind in a single world-embracing political organism. The term "world," orbis (terrarum), which imperial poets use freely in speaking of the Empire, is more than a mere poetical or patriotic exaggeration; it expresses the idea, the unrealised ideal of the Empire. There is a stone from Halicarnassus in the British Museum, on which the idea is formally expressed from another point of view. The inscription is of the time of Augustus, and the Emperor is designated as "saviour of the community of mankind." There we have the notion of the human race apprehended as a whole, the ecumenical idea, imposing upon Rome the task described by Virgil as regere imperio populos, and more humanely by Pliny as the creation of a single fatherland for all the peoples of the world. [Footnote: Pliny, Nat. Hist. iii. 6. 39.] This idea, which in the Roman Empire and in the Middle Ages took the form of a universal State and a universal Church, passed afterwards into the conception of the intercohesion of peoples as contributors to a common pool of civilisation--a principle which, when the idea of Progress at last made its appearance in the world, was to be one of the elements in its growth. 3.