XV. ‘And so it is probably with the universe, if it be indeed a living structure; in many places it contains earth, in many others fire, water, and wind, which are not forced out under stress, but arranged on a rational system. Take the eye; it is not where it is in the body owing to pressure acting on its light substance, nor has the heart fallen or slipped down [Sidenote: B] into the region of the chest because of its weight; each is arranged where it is because it was better so. Let us not then suppose that it is otherwise with the parts of the universe; that earth lies here where it has fallen of its own weight, that the sun, as Metrodorus of Chios used to think, has been pressed out into the upper region because of his lightness, like a bladder, or that the other stars have reached the places which they now hold as if they had been weighed in a balance and kicked the beam. No, the rational principle prevailed; and some, like eyes to give light, are inserted into the face of the Whole and revolve; the sun acts as a heart, and sheds and distributes out of himself heat and light, as it were blood and breath. [Sidenote: C] Earth and sea are to the universe, according to Nature, what stomach and bladder are to the animal. The moon, lying between sun and earth, as the liver or some other soft organ between heart and stomach, distributes here the gentle warmth from above, while she returns to us, digested, purified, and refined in her own sphere, the exhalations of earth. Whether her earth-like solid substance contributes to any other useful purposes, we cannot say. We do know that universally the Better prevails over the law of Stress. How can the view of the Stoics lead us to any probable result? That view is, that the luminous and subtle part of the atmosphere has by its rarity formed the [Sidenote: D] sky, the dense and consolidated part stars, and that, of the stars, the moon is the dullest and the grossest. However, we may see with our eyes that the moon is not entirely separated from the atmosphere, but moves within a great belt of it, having beneath itself a wind-swept region, where bodies are whirled, and amongst them comets. Thus these bodies have not been placed in the scales according to the weight or lightness of each, but have been arranged upon a different system.’