Some ideas harbor the notion that certain things are universal, say the desire to live and continue living.
However I don’t think universality ever actually exists. If only 99.99999% of a certain group is one way, that’s still not close enough, it’s not universal.
If it is not universal, and it’s actually somewhere on a gradient, then your definition is inherently arbitrary. It’s not that we can’t pick a set of standards and use them, of course we can, we all do. But that doesn’t make those standards any less arbitrary.
In the case of the desire to live, what about an elderly person in extreme pain that wants help dying?
Another is that humans are somehow separate from other animals. The gradient here is immediately obvious: Just what is a human? Whatever you want to define it as.
Babies, cavemen, and chimps, all show humanity is a gradient. If a definition is that slippery, not too much is gained by having it.
I think lines can be drawn. But drawing them without understanding that they are ultimately arbitrary is nonsensical.
Certainty is more dangerous than doubt.