When can previous rabbinic legislation be overturned? Avodah Zarah 36

On today’s daf TB Avodah Zarah 36 Rav and Shmuel disagree about the very essence of the prohibition of oil produced by non-Jews. Rav holds that the prohibition was legislated by Daniel’s court and Shmuel holds that it is only a halakhic consideration. “The mishna teaches: And their oil was originally prohibited but later permitted by Rabbi Yehuda HaNasi and his court. The Gemara cites a dispute with regard to the origin of the prohibition of oil. Rav says: Daniel decreed that oil is prohibited, and Shmuel says: The secretion of ritually impure vessels prohibits the oil that gentiles pour into them. The Gemara asks: Is that to say that all people are consumers of only ritually pure substances? Since it is common practice to eat ritually impure foods, why should the secretion of impure vessels render the oil prohibited? The Gemara emends Shmuel’s statement: Rather, the secretion of prohibited vessels prohibits the oil, as it absorbs the prohibited substances.” (Sefaria.org translation)

Shmuel raises a strong objection to Rav’s position. “But according to you, Rav, who said that Daniel decreed a prohibition upon the oil of gentiles, how can this be understood? Can it be said that Daniel decreed it, and Rabbi Yehuda HaNasi came and voided the decree? But didn’t we learn in a mishna (Eduyyot 1:5): A court cannot void the statements of another court, unless it is greater than it in wisdom and in number? According to Rav, how could Rabbi Yehuda HaNasi void a decree issued by Daniel?” (Sefaria.org translation)

As recorded in massekhet daf TB Shabbat 17b Beit Shammai legislated 18 prohibitions which Beit Hillel ultimately accepted. This legislation is unique as “doesn’t Rabba bar Ḥana say that Rabbi Yoḥanan says: With regard to all issues, a court can void the statements of another court, except the eighteen matters decreed by the students of Beit Shammai, as, even if Elijah and his court were to come and rescind them, one would not listen to him.”  (Sefaria.org translation)

Acceptance is another qualification when a later court can overturn an earlier court’s ruling. “Rav Mesharshiyya said: What is the reason that none of the eighteen decrees can be voided? Since the prohibition spread among the majority of the Jewish people, it cannot be voided. But with regard to oil, its prohibition did not spread among the majority of the Jewish people, and therefore it can be voided. As Rabbi Shmuel bar Abba says that Rabbi Yoḥanan says: Our Sages sat and inspected the matter of gentiles’ oil and determined that its prohibition had not spread among the majority of the Jewish people, and our Sages relied upon the statement of Rabban Shimon ben Gamliel and upon the statement of Rabbi Elazar bar Tzadok, who would say: The Sages issue a decree upon the community only if most of the community is able to abide by it. As Rav Adda bar Ahava said: What is the verse from which it is derived? It is the verse: “You are cursed with the curse, yet you rob Me, even this whole nation” (Malachi 3:9). This teaches that if there is the acceptance of the whole nation, yes, an ordinance may be instituted, but if not, no, the ordinance may not be instituted.” (Sefaria.org translation)

Tosefot ד"ה  וְהָתְנַן: אֵין בֵּית דִּין יָכוֹל  comes to the conclusion that any rabbinic legislation needs buy-in by the majority of the people. Without buy-in the legislation may be overturned. The acceptance has to be at the point of the original legislation. If it is accepted at the point of original legislation than only a court that is greater in number and wisdom may overturn.