In regards to your post, I'll try to go through it piecemeal and hopefully answer your question and clarify any potential misconceptions you might have.
The first part:
I want only Natural Law (Aristotle, Aquinas, etc) answers pointing to objective truth.
The statement above is a little bit "mixed mode," though easy enough to determine what was intended.
It's still very much a debate whether or not Aquinas' interpretation of Aristotle's natural law theory is accurate, but certainly both theories of natural law are theories of ethics. And, in that context, I'm understanding you use 'objective' in the term objective truth to be "morally objective" (eg "murder" is wrong.") Out of context, in a conversation about philosophy, if you used the term "objective truth" I would think you meant "there are no squared circles" or something along those lines.
Your comment clarifies this a bit:
There's a poem by G.K. Chesteron, I believe (at least I read it in
Gilbert magazine) called "The Poetry Should Rhyme" I don't remember
most of it and I can't find it right now, but I want to know if the
word "Should" in this case indicates an objective moral truth
In common language, people use the word 'should' in many circumstances to indicate just the opposite of objectivity - moral or otherwise.. When you want an audience to know you mean "morally/dutifully obligated" you'd use "ought."
If Chesteron did mean "ought" instead of "should", I think he'd have a hard time rationalizing this standpoint under any normative moral framework, primarily because he'd have to:
- Prove that poetry (or the process of creating poetry?) was a moral object/activity - i.e. "a code of conduct that, given specified conditions, would be put forward by all rational persons."
- That in order for poetry to be moral (and not immoral) it must rhyme.
Otherwise, the statement is essentially meaningless from a moral standpoint; no different from "tables ought have legs."