[3] It is right, therefore, for those who are investigating the question whether the world is perishable, since the two words, "corruption," and "the world," will be in continual use, first of all to investigate the precise meaning of both expressions, in order that we may know what is now signified, and what has been ordained. And we must enumerate, not indeed everything which is signified by those words, but so much as is useful for the purpose of our present instruction.