XXVIII. ‘As to the “stone” which some assume to be outside the world, it is not easy to form a conception of it, either as at rest or in motion. For how is it either to remain at rest, being weighty, or to move towards the world, like other heavy bodies, being no part of it nor reckoned in with its substance? Earth embraced in another world, and attached to it, need cause [Sidenote: D] no difficulty, when it does not part from the whole because of its weight, and shift hitherwards, since we see the natural strain by which each of the parts is held in its place. For if we look, not to the world but outside it, to get our conception of “below” and “above”, we shall find ourselves in the same difficulties as Epicurus, who made all his atoms move to places under our feet, as though either the void had feet, or infinite space permitted us to conceive of “above” or “below” within itself! Hence, again, we must feel surprise at Chrysippus, or indeed be quite at a loss as to what possessed him to say that the world has been settled “in the middle”, and that its substance, having occupied this middle place from all eternity, [Sidenote: E] works therewith for permanence and in fact for indestructibility. These are his words in the Fourth Book of his work on “_Things Possible_”, where he falsely dreams of a middle of the infinite, and assigns, still more preposterously, to that non-existent middle the cause of the stability of the world; and yet he had often said in other works, that substance is controlled and maintained by the movements towards and away from its own middle point.