From what I understand, that was a gezera from the Talmudic times in which they counted poultry as "meat" even though it is really not meat, because of the worry of mixing it up with meat and accidentally eating meat with cheese instead of chicken with cheese. One of the tannaim states his opinion that we can eat chicken with dairy, but his view is rejected by the Talmud. Things back then were different, before the times of packaged foods, etc, so I'm not sure exactly what the issues were, but in any case, we don't extend that gezera (as with Talmudic gezerot in general) to include more than what the rabbis of the Talmud included as far as I know.
I'm a little bit confused with what you wrote, KWRBT. I know that the Rabbis of the Talmud for some reason decided to canonize poultry as meat and it's signed and sealed, so irregardless of how we treat soy "meat" or soy "dairy", if a Rabbi says it's ok, it's ok.
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They didn't turn chicken into meat. They simply prohibited eating of chicken with dairy, as if it was meat, even though it is not meat. I don't think anyone claims that the rabbis turned poultry into meat. It was a fence around the prohibition to eat meat with dairy. They included chicken with that. Added it to the prohibition.
I didn't understand the part that I put into bold. Could you clarify that question?
On another issue. while the words of the written Torah can never be changed, what about rulings of Rabbis in the past? Can a ruling that is written in the Talmud be reversed, changed, or revised? Or once it's written, it's sealed for good?
This touches on a huge machloketh. In general, the traditional approach to halacha (which the deform movements separated themselves from and do not adhere to) is that the Talmud is binding, and all halachic authorities accept that as the framework for halacha. Varying interpretation of the Talmudic sources within the realm of plausible readings and possible understandings will come to underline halachic disputes, while no one will come to say a ruling in opposition to the Talmud itself which operates as a canon. But when there was a rabbinic gezera from the Talmud, based on reasons given by the rabbis enacting it there, and these reasons (whatever they were) are no longer relevant or no longer existing conditions in today's world, the question becomes, does the gezera remain in force, or can it be changed because the reason(s) it hinged on are no longer valid. That is a classic machloketh between the Rambam and Raavad. The Rambam says the gezera stays even if the original reason is no longer valid. I admit I have difficulty understanding that ruling. The Raavad says since the reasons no longer apply, neither does the gezera. However, this refers specifically to a derabanan gezera with reasons attached to it. The drashoth that underline Deoraita prohibitions and deoraita rulings are not negotiable except by a Sanhedrin, and even that is actually a difference of opinion, whether a future-formed Sanhedrin will have authority to contradict the Talmud... The Chazon Ish argued strongly against that when proposed by Rav Elchonon Wasserman. That the Oral Torah has not remained Oral and was put into writing greatly complicates this question. Without a Sanhedrin I think there is no question that we do not contradict the Talmud's halachic rulings (with possible exception for conditional gezerot), and I believe the Rambam elaborates on the methodology for psak halacha in which he includes multiple Talmudic sources being weighed against each other including Yerushalmi, Bavli, Sifre, Sifra, Tosefta, Braitot. However, some chachamim focus more strictly on Bavli and some later codifications of halacha including Shulhan Aruch.
Just as a tangent but this might be interesting, Lehavdil, the Deform movements came to reject Talmud in general and think they can cherry pick whatever they want for halacha. They don't operate within the rules of Jewish law and wanted to create Jewish law anew. For this the conservative scholars referred to a notion of "catholic Israel" which they invented and which has no basis in chazal or any sources before them. They explained this theological invention of theirs that the law is defined by what the Jewish people collectively decide to stop keeping or continue keeping. Therefore, if Jews start to eat shrimp (and they actually said this should go by majority of the observing community - how they define that is also specious) then shrimp has become permitted in Jewish law. Totally out of whack. That was a digression but I recently learned about that.