[4] [cap.2] The world, therefore, is spoken of in its primary sense as a single system, consisting of the heaven and the stars in the circumference of the earth, and all the animals and plants which are upon it; and in another sense it is spoken of merely as the heaven. And Anaxagoras, having a regard to this fact, once made answer to a certain person who asked of him what the reason was why he generally endeavoured to pass the night in the open air, that he did so for the sake of beholding the world, by which expression he meant the motions and revolutions of the stars. And in its third meaning, as the Stoics affirm, it is a certain admirably-arranged essence, extending to the period of conflagration, either beautifully adorned or unadorned, the periods of the motion of which are called time. But at present the subject of our consideration is the world, taken in the first sense of the word, which being one only, consists of the heaven, and of the earth, and of all that is therein.