Kol Yisrael Arevim - Im Yatza Motzee

The Gemara in Berachos 20b discusses whether women are chayav in Birchas HaMazon on a d'orayssa level or only midrabbanan. The gemara says it makes a difference in terms of whether a woman can be motzee a man in bentching. If the chiyuv of a woman is only midrabbanan her chiyuv is then a lower level of chiyuv than a man, who is chayav midorayssa. Thus, the woman would not be able to be motzee the man in the berachah. If, on the other hand, her chiyuv is mid'orayssa, then she could be motzee the man.

The Rosh there asks the following question. We know that by birchas hamitzvos there is a general rule of im yatza motzee. This means that if someone already was yotzei a mitzvah, he can still be motzee his friend in the same mitzvah. So, even if someone was already yotzei the mitzvah of kiddush, for example, he can still recite kiddush for his friend who wasn't yet yotzei. This rule is based on the inyan of arvus. Kol Yisrael Arevim tells us that every Jew is a guarantor for his friend. Thus, if a Jew hasn't yet fulfilled his mitzvah, it is as if all Jews haven't fulfilled their mitzvah, and they can be motzee this Jew. Based on this the Rosh asks, even if a woman were only chayav in bentching midrabbanan, couldn't we just apply the rule of arvus and say that since the man is chayav it is as if the woman is chayav and she therefore could be motzee him? In other words, why can't the woman use the concept of arvus to allow her to be motzee the man. The Rosh answers that the concept of arvus doesn't apply to women. What exactly does the Rosh mean by this?

This issue is raised in the notes of R' Akiva Eiger in Siman 271 of Orach Chaim. The Shulchan Aruch there brings the halacha that women are chayav in kiddush the same way that men are. Even though kiddush is a mitzvah aseh shehazman gerama we employ the rule of kol sheyeshno bishmirah yeshno bizchirah to be mechayev them. The Taz there says that this would mean that a woman can even be motzee a man in kiddush because there level of chiyuv is the same (as per the gemara in Berachos 20b).

R' Akiva Eiger there brings a safek from the Dagul Mervavah. The halacha is that one can be yotzee kiddush in the shemoneh esrai of Maariv. So, if a man comes home from shul and his wife hasn't davened yet, how can he be motzee her in kiddush? Since he has already davened his chiyuv in kiddush is only midrabbanan (because the chiyuv to say kiddush al hakos is only midrabbanan if you've already been yotzei the "kiddush" aspect). The woman, who has not yet davened, is chayav on a d'orayssa level. Thus, her obligation is on a higher level than her husband's. If so, the only way that the man can be motzee his wife is through arvus. Yet, the Rosh said that there is no arvus for woman. The Dagul Mervavah therefore is unsure what the Rosh means. If the Rosh means that there is no arvus for women at all, so the man should not be able to say kiddush for his wife.

R' Akiva Eiger then writes that, in his opinion, the Dagul Mervavah is misunderstanding the intent of the Rosh. In reality there is no difference at all between men and women when it comes to arvus. The rule of Kol Yisrael Arevim applies equally to both men and women. Rather, what the Rosh meant, was that if women are not chayav in bentching m'dorayssa, so for that mitzvah there is not arvus for women. Meaning, that arvus only applies to mitzvos which the person at least has a theoretical obligation to perform. If someone was never chayav in the mitzvah, there can be no arvus for that person in that mitzvah. Therefore, R' Akiva Eiger distinguishes between bentching and kiddush. By bentching, if the chiyuv for women is only midrabbanan, so there is no arvus for women. However, by kiddush, where women are as obligated as men, so there is also arvus for women. Thus, even though the man has already discharged his obligation for kiddush, he can still come home and be motzee his wife because of the rule of Kol Yisrael Arevim.