All property is imaginary. A construct of laws. You could make an argument in principle for own body being inherently sovereign, but even then people argue about it with things like fetal rights / abortion / mandatory vaccinations / organ donors / etc., not to mention that abhorrent ideas like slavery have existed which prove that people can indeed claim ownership on another's body.
The idea that property's defining attribute is that you can "give it back" is a strange invention of yours. Property is, literally, entity-dependent rights, as opposed to universal rights. Often these are fairly exclusive, just 1 person or a small family unit of people.
So, yea, you're right: they don't believe in Intellectual Property. They also don't believe in the Tooth Fairy or Santa Claus. Do you?
Intellectual Property is real. It's defined strictly in law and loosely by social convention, which are the only places where any notions of property are defined. This disbelief in Intellectual Property is completely unlike disbelief in Santa Claus. They don't believe Santa Clause exists; they may or may not believe Santa Clause existing is a good thing. They don't believe Intellectual Property existing is a good thing, but they absolutely agree that it exists.
Just so you know, the world was doing just fine and progress was being made -- creative works flourished, inventions were born left and right -- for all of human history before the ridiculous idea of "everything must be owned" started being written into law, a mere ~300 years ago, not even a blip on the human timeline.
The world's also doing fine now. Creative works are flourishing and inventions are happening at an unprecedented pace, with a large bulk of them happening well within the last 300 years as you cite. That makes this a ridiculous argument.
Furthermore, things recognizable as IP law go back thousands of years. There are records of ancient Greeks using something like a patent (with a 1-year term). Actual things called patents in English go back over 600 years. It's not as new as you suggest.
Um, you do realize that money is a fictitious construct as well, don't you?
Nevermind, we're not even gonna get into that. I just wanted to point out that referring to something that is completely made up as "practical" makes no sense at all.
In one sentence you point out that money is fictitious, and in the next you say that it makes no sense to refer to something fictitious as practical. I conclude that, to be consistent with those two claims, you must believe that money has no practical use. Do you agree that this is nonsense?
The problem is that you are playing fast-and-loose with the definitions of fictitious, imaginary, and made up. Using one definition, I can agree that fictitious things like comic book characters have no real life utility in and of themselves, although media featuring those comic book characters -- and in fact, the shared *idea* of those comic book characters as a representational artifact -- does have utility. Using another definition of fictitious, I can agree that money is fictitious. But you can't use the same definition for fictitious in both of those statements without reversing position on one of them; it's insanity.
Furthermore, if completely made up things are not practical, then there can be no possible problem with IP law in practice because it's completely made up. I do not believe you really have trouble making sense of these statements.
This is my problem with the FSF movement and RMS in particular. A lot of the arguments contain a huge amount of newspeak redefinitions or recastings of terms and acronyms and a) often they are nonsense, and b) even if they did make sense, you're spending all your time arguing semantics and not substance. It really, *really* doesn't matter whether IP stands for intellectual property or imaginary property, what matters is whether it's a good / effective idea in practice for either the majority of people, or for a minority of people that would otherwise be unfairly oppressed. There are well-known arguments on either side for both the majority case and the minority case. It doesn't really matter if it's GNU/Linux or Linux.
At no point in your post did you make a substantive stand against intellectual property. The closest thing was your claim that the world kept turning even without intellectual property, which is a very weak proposition, since the world will keep turning with or without intellectual property, as it would with or without medicine (we should choose "with"!) and with or without slavery (we should choose "without"!). There are actual arguments against, I've heard many of them before, some are even pretty compelling, but you are mostly spending your time insulting people for living in a matrix while you carefully redefine words to make the conversation impossible.