And if you look closely, people don't respect physical property to an exact degree. We share, we borrow, we lend, we lease, we sometimes treat human work as property, we're generally just lazy with our accounting of ownership. Which, you know, is kinda cool.
Anyway, you use the word arbitrary as if the arbitrary nature of the line makes the line meaningless.
In other words, there is nothing to invoke but natural law; and there is nothing behind natural law. Invoking natural law is _itself_ the means by which natural law gains utility.
What laws do you think are real? The only 'real' law is the Sparrow Predicate: what a man can't do, and what a man can do. _Everything else is built upon this._
That is to say, it is distracting and adding nothing sensible to point out that physical property, too, is an abstraction--it is merely an extension of the original point, which is: people don't buy into the intellectual property concept nearly as much as the physical property concept.
Well worth a read for what I think is a good way to understand the legal vs technical perspective of copyright.