Thank you very much.
I read what you wrote.

You write there an argument like this: Rabbi Shimon asks, from where would I be obligated if I had not been commanded? etc. And from here it is proven that, in his opinion, legal ownership in monetary law is something binding and not merely definitional.

But my question is that one could say Rabbi Shimon means that this is binding before the giving of the Torah, when the requirement was to conduct oneself according to reason alone; but after the giving of the Torah, only a command obligates me, according to the categories of reason. And therefore, if stealing from a gentile is permitted, then that too is correct and proper, even though it belongs to the gentile, because legal ownership in monetary law is only a definition.

How do we reject that possibility?

Just out of curiosity: according to your view, and according to the side I raised in my question, why did the Torah command us things that are understood by reason?
What is the point?

I don’t understand the question. After all, he says that stealing from a gentile is forbidden according to all views even today. You can see that in his opinion there is an obligation and not just a definition, and that this obligation exists even today after the giving of the Torah. In general, there is no reason at all to assume that the Torah comes to displace common sense. It comes to add to it, except where there is a contradiction. I discussed this in my article on “we require that Scripture repeat it in order to make it indispensable.” The same is true in my article on the status of logical inferences and reasoning (see there especially regarding the obligations of minors).

As for the question why the Torah commanded it, that is unrelated to this particular innovation. Why did the Torah forbid murder and theft? They were forbidden even beforehand (the Holy One, blessed be He, held Cain accountable for murdering Abel, long before the command). I have answered this in several places. My claim is that the Torah came to add a religious layer on top of the universal layer. From now on, someone who steals or murders violates both a legal prohibition and a religious prohibition. And there definitely are differences between the planes. One who murders indirectly, or under circumstances like “his sun’s end to come,” and the like, where he is exempt from the death penalty and seemingly did not violate the religious prohibition of “Do not murder,” is obviously still a murderer on the moral plane. But that is only with respect to the prohibition of murder, where Jewish law sets parameters different from those of the moral prohibition. With respect to property law, Rabbi Shimon argues that the Torah did not change the definitions, but merely overlaid them with an additional religious prohibition.

Yes, correct.

But the person I’m arguing with claims that Rabbi Shimon does not mean to say that stealing from a gentile is forbidden according to all views. Rather, he means that it belongs to the gentile, and according to the view that it is permitted to steal from a gentile, then it still is not “yours,” but it is permitted and proper.

And that is the plain meaning of the words: there is no right or authority to take from the gentile.

I explained why that is not possible.

I thought about it, and I think the Rabbi is right.
Since to say that there is monetary law and that it is not binding empties the definition of content,

because to say that something is “mine” and then say that anyone can take it from me or use it (for example, stealing from a gentile)—then it is not mine. So it is clear that Rabbi Shimon means to say that this too is binding. And besides that, there is the additional prohibition of “Do not steal.”
The proof from indirect murder is also good. Are there others?

By the way, what would Rabbi Shimon say about a gentile’s lost object, which one is forbidden to return?
After all, it is his.

There is also the proof from the fact that he asks: what is the obligation to obey? That is a clear proof in my opinion (the claim that after the giving of the Torah there is what I wrote is very forced. On the contrary, after the giving of the Torah there really is an obligation to obey, so what is the difficulty?).
As for a gentile’s lost object, in the simple reading the gentile himself gives up hope, because they do not return lost objects. One can discuss a gentile who lost it in a place whose majority is Jewish, and he relies on the fact that they will return it to him and therefore does not give up hope. But perhaps the parameters of despair themselves are different among gentiles, and for them ownership disappears from the moment the object is lost.
One can also say that the Torah permitted this outside the strict law in order to educate them to return lost objects (or simply because if they do not do this for others, why should we do it for them even if they deserve it).
By the way, that in itself does not mean that it is permitted to take the lost object; it only means that there is no obligation to return it. And in fact, according to Meiri, nowadays there is a full legal obligation to return even a gentile’s lost object, and that is proof for the latter thesis.