The first, most important, and most general conception is that of the division of the heaven into four quarters, which all of us that are capable of a general idea at all know as east, west, south, and north. The next is to arrange the several parts of the globe according to these points, and always to refer in thought any place mentioned to one or other of them. We shall thus get an intelligible and familiar conception of places which we do not know or have never seen. [Sidenote: General view of the geography of the world.] +37.+ This principle established as universally applicable to the world, the next point will be to make the geography of our own part of it intelligible by a corresponding division. It falls, then, into three divisions, each distinguished by a particular name,—Asia, Libya, Europe.[173] The boundaries are respectively the Don, the Nile, and the Straits of the Pillars of Hercules. Asia lies between the Don and the Nile, and lies under that portion of the heaven which is between the north-east and the south. Libya lies between the Nile and the Pillars of Hercules, and falls beneath the south portion of the heaven, extending to the south-west without a break, till it reaches the point of the equinoctial sunset, which corresponds with the Pillars of Hercules. These two divisions of the earth, therefore, regarded in a general point of view, occupy all that part which is south of the Mediterranean from east to west. Europe with respect to both of these lies to the north facing them, and extending continuously from east to west. Its most important and extensive part lies under the northern sky between the river Don and the Narbo, which is only a short distance west of Marseilles and the mouths by which the Rhone discharges itself into the Sardinian Sea. From Narbo is the district occupied by the Celts as far as the Pyrenees, stretching continuously from the Mediterranean to the Mare Externum. The rest of Europe south of the Pyrenees, to the point where it approaches the Pillars of Hercules, is bounded on one side by the Mediterranean, on the other by the Mare Externum; and that part of it which is washed by the Mediterranean as far as the Pillars of Hercules is called Iberia, while the part which lies along the Outer or Great Sea has no general name, because it has but recently been discovered, and is inhabited entirely by barbarous tribes, who are very numerous, and of whom I will speak in more detail hereafter. [Sidenote: The extreme north and south unknown.]