QUESTION: The Mishnah (73a) teaches that if an Eruv was made between five Chatzeros which are open to each other and open to a Mavoy, but no Shituf Mavo'os was made, the residents of the Chatzeros may carry from one Chatzer to another but they may not carry into the Mavoy. The Mishnah continues and says that if the residents of the Chatzeros made an Eruv and a Shituf, but one person forgot to join the Eruv, they may carry from one Chatzer to another, and from their Chatzeros into the Mavoy.

The Gemara asks that the first part of the Mishnah seems to contradict the second part. The first part of the Mishnah implies that an Eruv Chatzeros does not suffice in place of a Shituf Mavo'os. This follows the opinion of Rebbi Meir, because the Chachamim maintain that it suffices to have either an Eruv or a Shituf. The second part of the Mishnah says that if one person did not join the Eruv Chatzeros, carrying from one Chatzer to another is still permitted because of the Shituf made between all of the Chatzeros. This, however, does not follow the opinion of Rebbi Meir, because Rebbi Meir maintains that it is always necessary to have both an Eruv Chatzeros and a Shituf Mavo'os. The Mishnah is following neither the opinion of the Chachamim nor the opinion of Rebbi Meir!

The Gemara answers that the Mishnah is expressing the opinion of Rebbi Meir, who requires both an Eruv and a Shituf. However, in the case where only one person forgot to join the Eruv, Rebbi Meir agrees that Shituf alone suffices to permit carrying from one Chatzer to another Chatzer. Since the only reason why Rebbi Meir requires both an Eruv and a Shituf is in order to prevent the concept of Eruv from being forgotten (see Insights to Eruvin 71:2), if all of the residents -- except for one -- joined the Eruv, then there is no fear that the concept of Eruv will be forgotten, and a Shituf Mavo'os suffices.

If Rebbi Meir agrees that a Shituf Mavo'os alone suffices when an Eruv was made by all of the other residents of the Chatzeros, then why does the Mishnah say (in its final case) that if one Chatzer forgot to join the Shituf, then all of the Chatzeros are prohibited from carrying into the Mavoy? Here, too, Rebbi Meir should agree that the Eruv Chatzeros is sufficient to permit everyone to carry from the Chatzeros into the Mavoy. Since only one Chatzer forgot to join the Shituf, there is no fear that the concept of Shituf will be forgotten!

(a) According to some Rishonim, in the last case of the Mishnah in which an entire Chatzer forgot to join the Shituf Mavo'os, it is not sufficient that all of the other Chatzeros joined the Shituf. Since it is a large group that did not join, it is a situation which indeed can lead to the concept of Shituf Mavo'os being forgotten. In contrast, when only a single resident of a Chatzer forgot to join the Eruv Chatzeros, Rebbi Meir agrees that the Shituf suffices in place of the Eruv. (RITVA, TOSFOS RABEINU PERETZ, and RABEINU YEHONASAN)

(b) In his second answer, TOSFOS RABEINU PERETZ writes that the fear of the concept of Eruv being forgotten applies only when an Eruv Chatzeros is not made and a Shituf is made. Rebbi Meir prohibits relying on the Shituf because of the concern that the concept of Eruv Chatzeros might be forgotten. Technically, though, a Shituf alone should suffice. In contrast, the necessity to make a Shituf even when there is an Eruv is not just a decree made to prevent the concept of Shituf from being forgotten. Rather, there simply is no valid, logical basis for an Eruv to substitute for a Shituf, according to Rebbi Meir. Therefore, it is never possible to rely on an Eruv to carry into a Mavoy without a valid Shituf, and no one may carry into the Mavoy even if only one person forgot to join the Shituf.