If two principles command different actions (in some circumstance), then being free to choose principle B as a guide to one's actions means being free to ignore the different counsel offered by principle A. Which means that principle A is not binding upon you. And vice versa - being free to adopt A means that B is not binding on you. QED.
"Two plus two equals four" is communicated by language. As are all the truths of mathematics and physics Doesn't mean that they are merely inter-subjective (cultural) truths that people could choose to have otherwise by collective agreement. The medium of communication, yes; the message, no.Principles are culturally constituted, obviously. Language is culturally constituted. Principles are phrased in terms of language.
You've declined to provide a rigorous definition of what "reasonable" means. From your comment on the other thread, you may possibly think it refers to a consensus of the ideas held by reasonable people. But if you think that you know what that conventional wisdom is, and feel free to ignore anything that suggests anything to the contrary on the grounds that this is what a reasonable person would do, then you're not going to learn very much.That's what "reasonable" means.
Rightful possession is neither obscure nor abstruse. It is all possession except that achieved by wrongful acquisition.Who IS the "rightful possessor"? You have to make up some obtruse rule of "rightful possession" to establish what it involves.
I can't prove it, but rather suspect that every functional society there has ever been has laws against wrongful acquisition. Whether you call it theft or something else.
Which thereby meets your criterion (all or nearly-all societies) for a natural right - in this case a natural duty to refrain from thieving, and hence a valid natural law concept of rightful possession.
Sure, different societies may have different ideas as to what can be owned, who can own property, what is owned communally, etc. But they all have the concept of theft. Unless of course, you know different...
I'm influenced here by Nozick's concept of a just transaction. If you're the rightful possessor of a widget and give it to me as part of a consensual transaction (no theft, no fraud, no coercion, etc) then I become the rightful possessor. Simple as that.
Of the three, "liberty" seems to me the most ambiguous and culture-dependent.People may have different ideas about what the "right to life" or "right to liberty" might mean -- but "life" and "liberty" are clear concepts in any culture or language. "Property" is not.