I've had the displeasure to discuss this concept with people who doubt that they even have fundamental human rights. The concept to them, seems unprovable.

It seems a bit absurd to basically argue against your own ability to make a valid argument, which I think is encompassed in the position that human beings do not retain certain rights just by the nature of being...

But more upsetting is the contrary position: that there is no fundamental group of principles, and the only arrangement that exists is some kind of brute force.

I have a hard time understanding this viewpoint. If I showed up with one friend to murder and rob people who agree that might makes right, they would have no rational dispute for why I'm taking their life and their property. After all, they were outnumbered and overpowered. If they have no right to their person or property, then do they actually believe they are a moment away from justly losing them?