Continuing the previous post, aside from R’ Akiva Eiger’s question, the Rishonim reject Rashi’s assertion that someone who ate a k’zayis can be motzi someone who ate to the point of satiation because of a more fundemantal reason: from the Torah perspective, eating a k’zayis is the same as not eating, so what empowers such an individual who has no obligation to be motzi someone who does?
R’ Soloveitchik (Shiurim l’Zecerh Abab Mari vol. 2) suggested that the key to Rashi is understanding the concept of zimun. When three people become obligated to join together to fulfill the obligation of zimun, which in the days of Chazal was done by one person reciting all of birkat hamazon aloud and the others listening, it does not mean that each person has an individual obligation to bentch which is fulfilled by listening to the leader. Zimun means, rather, that the entire group as a whole is obligated in a single act of bentching. Just as elsewhere in halacha there exists a concept of a chovas hatzibur, a communal obligation, here too, the obligation is for one communal bentching, but instead of a tzibur of 10 the obligation rests on a group of 3 for zimun.
When each individual member of the community has a personal obligation to perform a mitzvah, e.g shofar, megillah, etc., we need to invoke the principle of shomea k’oneh to ascribe the mitzvah act of shofar blowing or megillah reading by one person to others. Shomea k'oneh means hearing someone blow the shofar is the same as individually blowing, but only if both the listener and doer are obligated in the mitzvah. This requirement stems from two seperate needs: 1) the need to define the act performed as a mitzvah, meaning here is an obligation to perform it; 2) the exclusion of a non-bar chiyuva from being motzi others. Ascribing the act of person A who has no Biblical obligation to fulfill a mitzvah to person B who does simply does not work.
The same is not true with respect to the mitzvah of zimun. Each member of the zimun is not obligated individually to bentch - all that is necessary is that the group as a whole share a single mitzvah act together. Shomea k'oneh is not needed; all that is needed is a shared mitzvah act. What defines a mitzvah act? Even though being obligated m’derabbanan does not make one a bar chiyuva on the Biblical level, it does suffice to define the bentching as a mitzvah act. According to Rashi, even a birchas hamazon recited by an individual who is only obligated to bentch m'derabbanan suffices to serve as the cheftza shel mitzvah to fulfill the communal chiyuv of zimun.
With this we can resolve R’ Akiva Eiger’s question. The gemara’s dilemma regarding a woman's obligation in bentching was not whether she is a bar chiyuva or not. The gemara’s question was whether since certain portions of bentching (such as the mention of inheritance of the land, or bris milah) do not apply to woman, is her bentching a mitzvah act parallel with that of a man or not? Rashi's opinion that a valid mitzvah act can be used to fulfill a communal obligation regardless of the level of obligation of the performer is irrelevant to the gemara's question of whether a woman's birkat hamazon constitutes a Biblically valid mitzvah act! In Brisker jargon, the gemara's safeik is in the cheftza shel mitzvah; Rashi is discussing the chovas hagavra.
R’ Soloveitchik has many proofs to this concept of zimun as a communal obligation, but I want to cut to the chase and get to what I’m stuck on. More to come…