As I say, I enjoy (to a point) such 'ultimate questions', but in the end one must simply plunk oneself down on one or another answer. For me, the 'real' problem is justly governing society.

How do we do that? We must first know what justice is.

In the end, knowing what justice is requires accepting what is perceived as human existence within material reality as the given field of inquiry. Otherwise, the issue of justice, the subject of which is interactions among material human beings, cannot arise. Moreover it is only within that perceived material reality that claims about knowledge can be externally validated/invalidated.

Beyond that perceived realm of existence, all claims become 'truth claims'. All are equally valid for those who choose to accept their validity and invalid for those who do not. It is all completely arbitrary. "Reason' can be employed to argue for one or another truth claim, but in the end (from the point of view of any other person, supposing they exist) one simply chooses to accept one truth or another.

Even within material reality anyone can reject any claim of knowledge about that existence. It is only within material reality, though, that knowledge as non-arbitrary commonalities of perception can exist. Justice, if it can exist, can only really exist pursuant to such commonalities.