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.