Pat Haba'ah Bekisnin, one blesses on it "who creates kinds of nourishment (borei minei mezonot)" and afterwards the three part bracha. If one ate a quantity that others would usually use to establish [a meal], even if he wasn't satisfied by it, one blesses "who brings forth (hamotzi)" and Grace after Meals. If in the beginning his intention was to eat just a little and blessed "who creates kinds of nourishment" and ended up eating a quantity that others establish [a meal] upon, he should bless Grace after Meals even though he didn't bless "who brings forth" in the beginning. And if he ate a quantity that others don't establish [a meal] upon, even though he is establishing [a meal], he should bless only "who creates kinds of nourishment" and the three part bracha, as his own mindset is irrelevant compared to [the mindset] of all people.
The Shulchan Aruch paskens that one has to bentch after eating Pas haBa'ah beKisnin, provided that others would fix a seudah on the same exact quantity (ืืืื ืืขืชื - both ืืืืืจื and ืืงืืื).
The Mishnah Berurah paskens the chidush of the Magen Avraham to bentch after eating some mezonos and something else when the two combine together to a normal kvius seuda.
The mishna lists the measures in which various substances are significant and generally stored. One who carries out straw in a measure equivalent to a cowโs mouthful is liable. The measure that determines liability for etza is equivalent to a camelโs mouthful. Because it is a coarser food, he must carry out a greater amount in order to be liable. The measure that determines liability for ears of grain is equivalent to a lambโs mouthful. The measure that determines liability for grass is equivalent to a goatโs mouthful, which is smaller than that of a lamb. The measure that determines liability for garlic leaves and onion leaves, if they are moist and fit for human consumption, is equivalent to a dried fig-bulk. A dried fig-bulk is the standard measure for human food. If the garlic leaves and onion leaves are dry, the measure for liability is equivalent to a goatโs mouthful. And none of these substances join together with one another to constitute a measure for liability because they are not equal in their measures. One who carries out a measure of foods fit for human consumption equivalent to a dried fig-bulk into a domain where carrying is prohibited on Shabbat is liable. And all those foods join together with one another to constitute that amount because they are equal in their measures. This amount is calculated without their shells, and their seeds, and their stems, and their bran, the husk that comes off of the wheat kernel when pounded, and their coarse bran that remains in the flour. Rabbi Yehuda says: None of the shells are calculated, except for the shells of lentils, which join together with the lentils to comprise the measure for liability because they are cooked and eaten with them.
The Mishna declares that things with different shiurim cannot combine. From here (and the occurrence of the same principle in Kodshim) the Chidah is questioning this ruling (in Birkei Yosef).
The Aruch haShulchan is questioning this ruling about counting non-mezonos items towards the amount of kvius seudah and seemingly paskens against โ with proofs.
The Aruch haShulchan refines his ruling and suggests strongly to take into consideration those opinions which want to count non-mezonos items as well, but probably only in an apparent seudah / ืืขืจืื-setting (otherwise he would contradict his earlier ruling).
The Magen Avraham brings compelling proofs from the original Gemara's lashon (see ืืืฆืืช ืืฉืงื: https://beta.hebrewbooks.org/tursa.aspx?a=oc_x977) and Tosfos / Smak (based on the fact that these are seudos of mitzvah, and as such, it would be unusual to have them on mezonos only).
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His proof offers a novel reading and understanding in the following Gemara and the whole concept of ืงืืืขืืช ืกืขืืื. It is surprisingly coincides with the above distinction of the Aruch haShulchan.
As the Gemara mentioned bread that comes as dessert, it now relates that Rav Huna ate thirteen substantially sized, sweetened loaves, three loaves per kav of flour, and he did not recite Grace after Meals because they were not genuine bread. Rav Naแธฅman said to him: That is hunger. One does not typically eat that much merely as dessert. Rather, over anything which is substantial enough to satiate and others base a meal upon it, one must recite Grace after Meals.
At first sight, the Gemara demonstrates the greatness of the amount consumed by Rav Huna, but in the Magen Avraham's novel reading, his feast had a "abstinent" aspect: that he was avoiding anything else besides mezonos (cf. ืคืช ืืืื ืชืืื). Others would probably extend this "meager" seudah with other items as well (besides decreasing its mezonos-quantity).
Rav Yehuda said to them: You are mistaken. Actually, it was stated that Shmuel said: The halakha is not in accordance with the opinion of Rabbi Mona. They said to him: But arenโt you, Master, the one who said in the name of Shmuel with regard to wafers: One may establish an eiruv, the joining of courtyards, to permit carrying in a shared courtyard and the joining of cooked foods, to permit cooking on a festival for Shabbat, with them and recite over them: Who brings forth bread from the earth. Why is that the blessing over those wafers? They too are sweetened bread that comes as dessert. He answered them: It is different there as he based his meal upon them, but here, where one did not base his meal upon them, no, he does not recite: Who brings forth bread from the earth.
Here the Gemara seemingly brings a contradictory ruling in which the amount of ืงืืืขืืช ืกืขืืื is not determined by the people in general, rather based on the individual (further emphasized by the ืชื ืื ืืคืื-like formulation).
In the light of all the above, the Aruch haShulchan's critique about the Magen Avraham's proof from here seems somewhat strange, since the Mg"A'sย explanation about the mention of Purim is much stronger connected to the topic than the assumption of the Ah"S about people eating ืืืื ืืืช on Purim only, in the time of the Gemara. (Also consider Ah"S 168:28 where he is making it clear that we have no information about there eating customs which deepens this questions.)