We are continuing in siman 3, discussing the concept of ain osim mitzvos chavilos.
The Gemara in Pesachim (102b) discusses a group eating a meal on Erev Shabbos which extends into Shabbos. Rav Yehuda holds that they need to make kiddush on a kos immediately, and should bring a second kos when they are ready for birchas hamazon. Rav Yossi holds that they can wait until the end of their meal, at which point they can take two kosos and use one for kiddush and one for birchas hamazon. The Gemara asks why two kosos are necessary according to Rav Yossi, and answers that it is improper to recite two kedushos on one cup, because of ain osim mitzvos chavilos.
The Gemara continues, and points out that if a person only has enough wine for one kos for both havdalah and birchas hamazon, they can use the one kos for both, and we do not appear to be concerned for ain osim mitzvos chavilos. The Gemara answers that such a case is an exception, because the person does not have a choice. The Gemara counters that when Yom Tov falls out on Motzei Shabbos, the same kos is used for both kiddush and havdalah (havdalah is made within the kiddush) and we are not concerned for ain osim mitzvos chavilos. The Gemara concedes, and explains that havdalah and kiddush both center on the kedusha of Shabbos, so they are considered one topic. Therefore, doing them together is not an issue of ain osim mitzvos chavilos. However, the kiddush and birchas hamazon are two different mitzvos entirely, so bundling them together upon one kos would be considered ain osim mitzvos chavilos. If a person has no other option, they may do it, but otherwise they cannot.
In many places, the kos for birchas hamazon of shalosh seudos is used for havdalah. Although it should be a problem of ain osim mitzvos chavilos, regarding shalosh seudos, borei pri hagafen is not made on the kos (see below). Since the mitzvah (the kos of wine) would otherwise be ignored, it is actually proper to use it for havdalah so that it is used for another mitzvah. If one would try to combine them, such that they would make havdalah immediately after benching, it would be a problem of ain osim mitzvos chavilos.
(We do not make a borei pri hagafen on the kos immediately after benching at shalosh seudos due to a safeik of whether it is muttar to eat and drink or not. Generally, one cannot begin eating or drinking on Shabbos after shkia until they have made havdalah, unless they began their meal (i.e., washed for bread) before shkia. However, it could be that the kos is considered connected back to the meal, so it would be muttar to drink it before havdalah. Due to the safeik, we do not make a bracha on it. )
Tosfos writes that it is not a problem to use one kos for both borei pri hagafen (a birchas hanehenin) and kiddush. However, regarding the kezayis for hamotzi and kezayis for achilas matzah on the seder night, Tosfos hold it would be an issue of ain osim mitzvos chavilos to eat them both from the same matzah. Therefore, Tosfos writes that one should eat a kezayis from the upper matzah for hamotzi and a kezayis for achilas matzvah from the middle matzah, as is the halacha. We see that Tosfos extends the concept of ain osim mitzvos chavilos even to birchos hanehenin.
The Mishnah Berurah grapples to find a source for the need to eat a kezayis when reciting a birchas hanehenin. Nonetheless, the halacha is in accordance with Tosfos.
In conclusion, we see that this seemingly esoteric concept of ain osim mitzvos chavilos has many applications to mitzvos which we do regularly, even to the proper procedure for taking out sifrei torah or making brachos.
- Making kiddush and havdalah on the same kos (such as on a Yom Tov which falls after shabbos) is not considered ain osim mitzvos chavilos because they are both of the same topic.
- However, kiddush and birchas hamazon are different topics, so one cannot use the same kos for both (unless they have no choice because they only have one cup of wine).
- On the other hand, the same kos is used for birchas hamazon and havdalah, but only because the kos for birchas hamazon would otherwise be ignored.
- Eating the kezayis for hamotzi and kezayis for achilas matzah on the seder night from the same matzah would be an issue of ain osim mitzvos chavilos. Therefore, they are taken from different matzos.