I have been learning through the answers and articles here and there is a very consistent method, for example the fast days article where the whole psak comes out of the exact definition of one word, shalom, in Rosh Hashanah 18b. I want to ask about the method itself.
1. When the Torah or the gemara gives a rule with a stated reason, is the stated reason always the exact edge of the rule? Meaning if the reason does not apply, the rule was never talking about that case at all, and a definition question is really just asking whether the case ever entered the category in the first place?
2. How do gezeirot actually work? When chazal make a gezeira they say the worry out loud, gezeira shema this or that will happen. Is a gezeira always measured by its stated worry, so ein lanu lehosif al hagezeira means nobody can stretch it past the reason they said? And how does that fit with the sugya in Beitza that gezeira l’gezeira lo gazrinan, and with the idea of lo plug, that chazal did not make distinctions inside their own decrees?
3. The mainstream sevara I hear everywhere is lo plug rabbanan, once chazal made the rule it applies across the board even where the reason is gone. If the reasons and definitions really set the edges, why do so few poskim pasken this way today? Is it a real machlokes about how halacha itself works, or does everyone agree in theory and they are just being machmir in practice?
4. Practically, when a person learns a sugya and wants to know the real halacha, how do they know where a rule ends? Read the reason and stop exactly there, or follow how the poskim applied it over the generations even when they went past the reason?
There really is no one rule that answers all of these questions. Sometimes the reason given by the Torah or Chazal defines the category itself, so if the reason is absent the case never entered the prohibition. Sometimes Chazal made a broader gezeirah and we say לא פלוג רבנן …. once they drew the line, they intentionally did not distinguish between individual cases. And sometimes extending a rule further would itself be גזירה לגזירה or adding something Chazal never prohibited. All of these principles are real; the difficult part is knowing which one applies in a particular sugya.
That is why you cannot create a mechanical rule saying either, “Whenever the reason disappears, the prohibition disappears,” or the opposite, “Once Chazal prohibited something, the reason no longer matters.” Both statements are wrong as universal rules. You learn from the language of the Gemara, the surrounding sugyot, how Chazal themselves applied similar rules elsewhere, and then the Rishonim and Poskim. This is one of the reasons a person really has to learn Shas as a whole. You learn where Chazal say lo plug, where they do distinguish, what constitutes a safek, when we refuse to add to a gezeirah, and how definitions operate. Those instincts come from seeing the principles used repeatedly throughout the Gemara, not from memorizing one slogan.
So I cannot give you “the method of the entire Torah” in one Q&A. If you give me a particular halachah …. a fast day, a gezeirah, a definition, a Shabbat issue …. then we can open the Gemara and see exactly what Chazal said, what category they created, where they drew its boundary, and why. That is the proper way to do it: not starting with a predetermined chumra or kula, but learning the particular sugya carefully enough to discover what rule Chazal actually made.