[21] And if the world also is destroyed, then it must of necessity be so either by some external cause, or else by some one of the powers which exist within itself; and both these alternatives are impossible, for there is nothing whatever outside of the world, since all things are brought together in order to make it complete and full, for it is in this way that it will be one, and whole, and free from old age; it will be one, because if anything were left outside of it, then another world might be created resembling that which exists now; and whole, because the whole of its essence is expended on itself; and exempt from old age and from all disease, since those bodies which are liable to be destroyed by disease or old age are violently overthrown by external causes, such as heat, and cold, and other contrary qualities, no power of which is able to escape so as to surround and attack the world, all those being entirely enclosed within, without any part whatever being separated from the rest. But if indeed there is any external thing it must by all means be a vacuum, or else a nature absolutely impossible, which it would be impossible should either suffer or do anything.