Aristotle ends the Ethics by announcing the Politics. The last chapter says that knowing what the good life is does not produce it, that habit is formed by law, and that the enquiry must therefore continue into constitutions. The Politics opens where that sentence stops. The dependence runs the whole way through. The Politics assumes the Ethics' account of virtue as a settled habit and of happiness as activity rather than feeling, and uses it as the standard by which a constitution is judged: a good state is one under which its citizens can actually live the life the earlier book described. Read alone, either half loses the argument that connects them.