Before turning to the various regions of the known world—the _oikoumene_, as the Greeks called it; the _orbis terrarum_ or _habitatio_ of the Romans—something must be said concerning theories about the _oikoumene_ as a whole, about its center, and about Paradise and the four rivers of Paradise. It was usually supposed that the _oikoumene_ itself occupies a relatively restricted part of the surface of the globe. The words of Seneca to the effect that there is only a short distance from Spain to India imply that the known world must stretch out over much more than a half of the circumference of the sphere.[1175] Though these words were often read in our period, scant attention was paid either to them or to the Arabic interpretation of Aristotle’s similar theory until a later date. Roger Bacon’s specific explanation that the _habitatio_ extends around much more than half the earth’s circumference represents an opinion that was exceptional.[1176] The majority of the thinkers of the twelfth and early thirteenth centuries who speculated on the subject at all were probably under the spell of the theory fostered by Macrobius, which made our habitable portion of the earth one of four similar regions separated from each other by two oceans.[1177] This undoubtedly was the view most widely accepted, but in addition the idea was perhaps already being propounded early in the twelfth century that the lands of the known world form merely a small portion of the surface of the terrestrial sphere emerging above the surface of a larger, enveloping sphere of water.[1178] THE “OIKOUMENE” DIVIDED INTO THREE PARTS The writers of the Crusading age were unanimous in dividing the _oikoumene_ itself into three parts, Asia, Libya (or Africa), and Europe. Bernard Sylvester said: “In two parts the ether, and likewise in two parts the air, but in three parts you are to understand that the land is divided,”[1179] almost as if a tripartite division of the lands were in accord with a law of nature. This division was inevitable in view of what was known of the arrangement of lands and seas. Orosius,[1180] however, had spoken of certain writers who would split the known world in two, making Africa a part of Europe “because of its small size” and making Asia as large as Africa and Europe together. Those who had preferred to conceive of Africa as a separate continent, he had said, did so not on account of its size but because it is cut off from Europe by an arm of the sea. These words of Orosius were quoted by Otto of Freising[1181] and by Gervase of Tilbury.[1182] The theory that Asia is equal in size to Europe and Africa put together is reproduced by the author of the _De situ terrarum_,[1183] and upon it was based that symmetrical division of the world’s surface which we find depicted on the so-called T-O maps of the early Middle Ages.[1184]