[nooneelse]

You are shifting all the work that the criterion was meant to do into the term "meaningfully". Without elaboration on the application of that term, this is of no help as I used a very common standard of "meaningful", "having meaning". A question has meaning, even when the answer is obvious or the answer to a "who" question is "no one". Example: "Who has been to Mars?" is a clearly meaningful sentence.

The issue at play later is: does person A have a right to liberty. Warrenwilkinson's criterion for it not being a right was the ability to append "At whose expense?". I even made that a more reasonable criterion, by implying the further requirement that, in order for a hypothetical right to be considered not a right, it had to come at the expense of someone else (the elaboration used by many other people in better statements of similar criteria, and seemingly the underlying consideration really at play in the two lists warrenwilkinson gave). The objection to even this elaboration was that in the case of a society with slavery, the hypothetical "right to liberty" would obviously come at the expense of slaver owners.

You introduce the interesting symmetry constraint, as charitably as I can gather, intending to use the symmetry between the slaves and owners. No help really, as the owners might as easily conclude that they don't have a right to liberty, they simply have liberty itself. If you landed on an island with slavery on-going, does the criteria proposed have any teeth?

There is, I think, a sharper edged point than this merely amusing slavery objection, but I've mentioned it in other threads already.