When you argue about values, you use demonstrative rhetoric, not deliberative. If you rely on a cosmic authority— God, or Bono— then the audience has no choice to make.

Eternal truths will answer these: Is there a God?

Is homosexuality immoral?

Is capitalism bad?

Should all students know the Ten Commandments?

In each case the argument has to rely on morals and metaphysics. And it takes place mostly in the present tense, the language of demonstrative rhetoric. It can be particularly maddening in a marital dispute, because it comes across as preachy. (Demonstrative rhetoric is the rhetoric of preachers, after all.) Besides, it is far more difficult to change someone’s values than to change her mind. After all, eternal truths are supposed to be … eternal.