I don’t quite understand what you mean. How would you propose figuring out if IP really does follow from principles of property rights?
The same way we establish what rights ought to be in general. By looking at what kind of principles protect man’s freedom to engage in self-sustaining action, on whatever scale his skill and intellect allow for.
I said this above too, but it was in a way too long post, so I’ll just post it again here: The question is does IP serve that purpose, or stifle it? I think that’s an easy question to answer, and Ayn Rand answered it by noting that the most fruitful, important kind of self sustaining actions are intellectual in nature. We must protect them before anything else, if we are to thrive.
My thought experiment is meant to be useful in the sense that if one has a strong case for IP, then figuring what Franz may or may not do about Franz should be easy. Since “it’s just obvious” is not acceptable, the questions I started out with in this post are also important. The point isn’t to deduce an answer from a single case, but to make it easier to think about philosophical questions.
Ok, I think I see what you mean. So, then, the thing to show in this case is that Franz, by designing a piano and sharing his design with others, for a price, is engaging in life sustaining action. But, if Wolfgang is just allowed to buy Franz’s piano, copy it, and then build the same design without Franz’s permission, then Franz’s ability to design pianos for a living is stifled. Wolfgang is not engaging in life sustaining action, he is in fact a parasite, taking away from Franz’s ability to create.
From this case, and cases like this, through induction (not deduction), one can then establish that such a state of affairs stifles innovation. One can also establish that a system of government which allows Franz to protect his design, and sell it exclusively, would allow creative people like Franz to live a fully creative, productive life.
That is the way in which the example is relevant. Obviously, that’s not the end of the story. There’s still a long way to go towards establishing that the right way to protect Franz and people like him is the kind of framework of laws we have in place today protecting copyright, patents, etc. But we did at least establish that the desired outcome is to allow Franz to reap the full benefits of his intellectual work, and that the only way that can happen is if he has the exclusive right to sell his pianos (but without abusing his ownership of the design to achieve some goal other than just this specific one - i.e. without gaining the ability to block innovation by buying up a million patents and sitting on them).
Then, we must design a system of laws protecting intellectual property that achieves this desired outcome, but does nothing else.