True, many interpret Rabbi Shimon as saying that the legal system determines only the boundaries of ownership, without prohibitions, and only Jewish law determines the prohibition. But that is a mistake, and I have already pointed this out in several places. For Rabbi Shimon there is a legal prohibition, not just a legal determination of ownership.

Thank you, you settled for me intuitively the principle of deriving from a legal norm to a halakhic one—that was really what I meant. But for the sake of the issue, how would he connect the systems in an essential sense? What assumption would he have to add? It still seems to fall into some kind of fallacy (I just don't know what to call it).

I don't know a name for it. In principle, you could argue that just as one cannot derive a norm from a fact, so too one cannot derive a norm of type A from a norm of type B. These are two different categories that do not speak to one another. With regard to Jewish law and morality, I actually do think this way, but many others think differently.

How can one say about something that there is a legal prohibition on it?

After all, law never determines truth and justice; it only aspires to them, with clear awareness that in a significant percentage of cases it will fail at that (like every civil legal system throughout history).
Unlike that, Jewish law is free to do and determine whatever it pleases—an instance of “the guards were removed, the locks were broken, the chains were taken off, and the gates were uprooted.”

Therefore it makes sense to say that there is a legal determination of ownership, and on top of that laws are applied.
But what is the logic of saying that there is a legal prohibition? What does that even literally mean?

Is that a poem or a question?

Hilarious. You should perform in nursing homes. You'll be a hit.

What's unclear??
"Legal truth" has nothing to do with—and nobody ever claimed—it is objective truth.
Accordingly, one would not claim to apply a legal prohibition, because the law itself is not sure of its own justice. One can apply a legal prohibition to whatever one wants whenever one wants, even if we made a mistake. So I didn't understand what you explained in Rabbi Shimon's words.

The "poetry" is a quote from Rabbi Lichtenstein about the virtue of Brisk.

It may be that in your nursing home I'd be a hit. What I wrote was not a joke but completely serious. In your words there wasn't a question, only a poem. And that is true of your "clarification" here as well. What does certainty have to do with anything? Is there certainty in Jewish law? What does all this have to do with our discussion? It isn't even a poem, just a pile of meaningless words.
As I said, even what you wrote here is still not at all clear to me, and again I see no question. Please formulate more precisely what you are asking, define terms, and explain your basic assumptions. Good luck.

You wrote: "For Rabbi Shimon there is a legal prohibition, not just a legal determination of ownership."

I claim:
1) Jewish law is not bound by anything, and therefore whatever it rules is truth.
2) Law—by definition—does not have truth, only an aspiration to truth.

My question was an innocent one (I am really and sincerely trying to understand what you meant): how can one say about something that there is a "legal prohibition" on it? After all, in law I have no confidence that I am right, since perhaps there are false witnesses, or perhaps one of the judges took a bribe, etc., etc. Therefore the opposite view from yours makes sense to me: that the legal system determines only the boundaries of ownership without prohibitions, and only Jewish law determines the prohibition.

I am a good, kosher Jew who wants to drink from the river of your wisdom and understand your opinion. That, and nothing more.

P.S.
As for the "poetry," I'll explain briefly.
Rabbi Lichtenstein said on several occasions that the virtue of Brisk is that this method "removed the guards and uprooted the gates," etc. (the gates of casuistry and intellect). Rabbi Shimon Shkop was a rosh yeshiva in Telz, and there he developed his teaching.

The Sura and Pumbedita of 2–3 generations ago—as is known—were Brisk and Telz.

I amused myself by using, in my attempt to understand Rabbi Shimon, expressions that were said about his great miracle—Brisk.

You'll say it's not amusing? Fair enough. At least you've gained this: if you ever want someone to tag along on a performance tour, you've got my email….

I truly cannot understand where your strange assumptions come from. And I still cannot understand why, even if they were correct, you think you have found some difficulty in all this for what I said.
1. Law aspires to truth exactly like Jewish law. And exactly like it, it does not necessarily arrive at truth. We do our best, and the conclusion is what binds us.
2. Jewish law also aspires to truth, and does not always reach it. The conclusion is what binds, as above.
3. Both Jewish law and law are valid and binding, whether they hit upon the truth or not. On the contrary, law has an advantage over Jewish law, since in Jewish law there is a rule about one who errs regarding the commandment to heed the words of the sages (see the beginning of tractate Horayot), meaning that if it is a mistake there is sometimes no obligation to obey, whereas in law that is not so. (According to your strange view, one would have to conclude from this that law specifically does not aspire to truth. Nonsense, of course.)
4. Even if you were right about everything—so what? Because there is no certainty, therefore it is not valid or not binding? Where did you get that nonsense from?

On my performances, I'll try to take someone along whose words I understand. Otherwise I won't laugh at his jokes either, and that's too bad.