“It’s not a physical truth about the universe.”

I 100% reject any attempt to appeal to something that is “useful” rather than “philosophically airtight.” That way lies utilitarianism, and a great horror that is.

What does philosophically airtight even mean for legalistic frameworks? “Consistent given the axioms”? What happens when someone rejects the axioms?

Reject the axiom, eject from helicopter.

But more seriously, the argument there is one must establish the axioms as just that, axiomatic. And if they are, than someone rejecting them is rejecting reality, and you couldn’t reason with them anyway. As a rule, most of the time folks implicitly acknowledge the axioms in when attempting to reject them.

(A Spontaneous Order is a good book on this.)

Axioms aren’t physical reality though, that’s the point, they’re chosen via a social process that requires ongoing negotiation.

Whether grazing your sheep on otherwise untouched land confers ownership vs. planting crops on land vs. regularly setting fire to the land vs. burying ancestors on the land vs. etc. etc. have all been axiomatic to different groups at different times, and these questions can’t be answered by in some abstract sense, they require getting everyone in a room and thrashing it out until agreement is reached.

I feel we are maybe meaning different things about axioms. I don’t agree that they’re chosen via social process at all. An axiom is true even if everyone attempts to say it is not, that’s what makes it axiomatic. 

That all of those things HAVE been debate doesn’t mean there is not a conclusive truth, but we’ve moved beyond things. We are now discussing far more than an axiom of ‘property rights exist,’ but rather are now defining what those property rights ARE. The axiomatic truth is that ownership exists, not necessarily the minutia of what that ownership entails.