One point I don’t think anyone’s brought up in more than passing yet is empathy. It’s pretty simple- a bit like the contract theory without the inherent belief in evolutionary competition being the root of all. Essentially:
I can empathize with a human I have never met- even one marginally stupider than I could imagine being- pretty uniformly across the board. Yes, it is easier to empathize with someone closer to my intelligence level, but the differences are minimal or negligible in comparison to, say, an ostrich. I hear of them suffering and/or dying and am saddened even if I have no personal connection to them.
I can’t empathize with an insect. I know that millions are casually and in ways that, if they were sized proportionally to me, would be unimaginably horrific/gory and am not moved.
Now, to move into grey areas:
I can empathize to a considerable extent with a domesticated dog. His owner dies, he grieves; if his owner is a friend of mine, I may even grieve too, and some would say in startlingly similar ways.
I can barely, if at all, empathize with a cat. His owner dies, his first concern is where to find a new two-legged slave. Therefore I consider a dog to be more of a person and entitled to more rights than a cat.
After thousands of years of genocides and conquering we have begun to formulate and agree on a universal set of rights within humanity as repetitive experience teaches us that many if not most of the experiences with which we empathized in close friend/family bonds are universal to the species. We’ve still got pretty far to go, but we’ve moved somewhere.
More complex elements of this idea, such as the rights of very simple humans, animals, humans who have committed atrocious crimes, humans suffering from psychosis, juvenile humans, and even those influenced or manipulated by fellow humans are to this day still being debated and defined. IANA expert but I would not be at all surprised if in the end these decisions all came down to what extent the general populace can understand and empathize with each case.
Is it right to base everything on empathy? Possibly, as the very nature of that criterion could force us to use it responsibly. Most of us have heard it’s much harder to commit a war crime while looking your victim in the eyes; if we do our best and apply ourselves to comprehending and “feeling” the situation as much as possible, then causing undue harm to any creature which perceives and endures that harm will sicken us and that guilty response will keep us in check.