The first Mishna of the massekhet (2a) taught that a “replacement” kohen was appointed in order to ensure that there would be a High Priest who could perform the Yom Kippur service in the event that something were to happen to the Kohen Gadol.
What happens to a kohen who replaces the High Priest when the High Priest recovers and can once again serve in the Temple?
The Gemara (12b) brings a tosefta in which Rabbi Meir and Rabbi Yosei disagree about such a case. According to Rabbi Meir, the original Kohen Gadol returns to his position, and his replacement continues to keep all of the rules and regulations of the Kohen Gadol. Rabbi Yosei rules that the original Kohen Gadol returns to his position, and that his replacement can no longer serve in the Temple at all. He cannot serve as the High Priest because the presence of two Kohanim Gedolim would lead to enmity between them; he cannot return to the position of a regular kohen because of the principle ma’alin ba-kodesh ve-lo moridin – we raise people to higher levels of sanctity, but do not bring them down.
The Gemara on our daf rules like Rabbi Yosei against Rabbi Meir, noting that even Rabbi Yosei agrees that if he were to perform the Temple service it would be acceptable after the fact, and that he could serve as the High Priest upon the passing of the Kohen Gadol.
Several questions are raised by the commentaries regarding the Gemara’s ruling. Tosafot points out that it is rather unusual to find the Gemara offering a ruling on a topic that is not pertinent in the contemporary period. This ruling, after all is hilkhita le-mishiha – a rule applicable in the Messianic age. The R”i ha-Lavan asks why there is any need to state a ruling like Rabbi Yosei, given the Talmudic principle that we follow Rabbi Yosei in all of his arguments with Rabbi Meir.
With regard to the first question, some suggest that this discussion does have contemporary application, specifically in a case where a community leader is incapacitated and replaced, only to recover his abilities. How should he and his replacement be treated? Perhaps we can derive some direction from this discussion in the Gemara. Some attempt to answer the two questions by turning them against one another. Since the question is not one that applies to a real situation today, we cannot apply the normal rules of following Rabbi Yosei against Rabbi Meir, forcing the Gemara to explicitly state that the ruling is like him.