In the future, don’t send details of the question in a different thread.
You wrote:
Example: when some person tells me, “Bring me the keys,” how can I know that his meaning of the word “keys” is my meaning, and that his meaning of the word “bring” is my meaning, etc.? Maybe he interprets the word “bring” the way I interpret the phrase “dance in the air,” and so on…

Doesn’t the Rabbi hold that this comes in the wake of Platonic conceptions at the level of definitions? Whereas context and experience confirm it. (But they are not themselves the actual reason.)
By the way, is the philosophers’ chestnut a difficulty regarding our understanding of the world? Or does it have no practical significance because it deals only with the subjective perception of things, even though in the end they do have a source in the world as such?

I don’t understand the question. The philosophers’ chestnut may have no practical significance (and therefore there is no way to decide it), but it is an interesting question. Even if things have a source in the world itself, the picture reflected in us is also something that can be discussed. Beyond that, one can of course also ask the same thing about the ontic source of the things: is there indeed the same thing in the world itself when I report something red and when you report it?

Rabbi,
I vaguely remember that somewhere you wrote that the network of connotations and associations greatly weakens (for reasons of simplicity, not because of a principled difference) the possibility that the chestnut holds true in a very broad sense—for example, maybe what I see as a green tower others see as a tomato pie. I couldn’t find it now. If you wrote about this, could you point me to it?

I didn’t understand what you’re looking for. Indeed, a green tower could be perceived by someone else as a tomato pie.

I remember an argument that weakens that possibility, and doesn’t just dismiss it as skepticism.

That sounds interesting—what is it? Maybe the Rabbi remembers?
If I recall correctly, I remember that at the time he argued this from the angle of emotions, but later retracted it.

I remember that I raised a possibility based on connotation. For all of us, the color black creates sadness and depression, and a bright color creates the opposite. Seemingly this is evidence for something objective in the perception of colors. But of course this is rather weak evidence.

About a permutation of colors alone, it’s really hard to challenge it positively. But if the whole picture of the world is completely different—I see a mosquito and he hears an Andalusian choir; when I think he’s using his hand to scratch, he’s actually taking off in a helicopter; he says “hammer” to me and I hear “toilet paper.” And somehow the world keeps functioning and the illusion continues, because there is a vast action-preserving mapping that keeps each person inside his own bubble. Theoretically it seems possible, but much more complex, and therefore less plausible even than the idea that my consciousness is the only one that exists. But I don’t know.

[“action-preserving mapping” should read “operations-preserving mapping.” And I mean that even mathematically, the more operations that are defined in a space, the harder it is to find a homomorphism between the spaces.]

“How can I know that I know the correct meaning of a certain word?”

There is no “correct” meaning to a word. There is an accepted meaning, a customary one. A word is a means of social communication. Not Platonic philosophical nonsense.

Rabbi, can’t one see from neuroscience some kind of correlation between people?
For example, if “calming” music like classical music creates in us a pleasant feeling, and in parallel we observe in the brain a state that seems relatively “soft” and less active than rock music, isn’t there reason to assume that in the other person too rock music will not arise in consciousness as we perceive it, but rather also something soft?
This would not completely answer the philosophers’ chestnut—for example, the question of exactly what kind of sound or color would arise for him. But it would answer the claim that the phenomena for him would be “soft.” So there is no reason to think that it is literally the reverse of how it is אצלנו. (What is classical for us is rock for him, and vice versa).

That is somewhat like what I wrote about moods. And I already noted there that this is rather weak evidence.

Yes, only here there is “scientific” confirmation that can serve as a certain measure of the scope of the phenomenon, so I don’t think it is quite so weak, whereas in your case the confirmation was subjective in itself. It’s an analogy that sounds plausible: states that are very active would produce very volatile emotions, and vice versa. (Although that does not prove an identical match to the emotions, only the general scale of the matter.)

By the way, above you presented a possibility that the thing in itself is not only perceived differently by each person, but is itself different. Does that too fall under the philosophers’ chestnut? To me that sounded like ordinary skeptical claims, which as far as I know you say do not trouble you, right?