OPINIONS: The Mishnah discusses the amount of bread that constitutes two meals, with which an Eruv may be made. Rebbi Shimon says that two meals are comprised of a volume of bread equal to 8 eggs. Rebbi Yochanan ben Berokah says two meals are comprised of a volume of bread equal to 6 eggs.

This argument has a number of other practical ramifications. For example, if a person eats one meal's worth of Pas ha'Ba'ah b'Kisnin (see Insights to Berachos 42:1), he must recite ha'Motzi and Birkas ha'Mazon as he must do when he eats bread. According to Rebbi Yochanan ben Berokah, an amount of Pas ha'Ba'ah b'Kisnin equal to 3 eggs constitutes a full meal, while according to Rebbi Shimon, an amount equal to 4 eggs is necessary. (See Mishnah Berurah OC 168:24, however, who cites some authorities who do not compare the quantity of food necessary for an Eruv with the quantity of food necessary for other laws.)

Does the Halachah follow Rebbi Shimon or Rebbi Yochanan ben Berokah?

(a) We know that food can become Tamei with Tum'as Ochlin. Whenever the Gemara discusses Tum'as Ochlin, it says that food the size of an egg ("k'Beitzah") is Mekabel Tum'ah. Rashi (83a) writes that according to Rebbi Yochanan ben Berokah, the amount should not be the size of one egg, but 3/4 of an egg (half of a half, or a fourth, of a loaf of bread the size of 3 eggs). Why does the Gemara always say that the amount is the size of one egg? TOSFOS (83a, DH v'Chatzi) says that whenever the Gemara says that Tum'as Ochlin applies to food the size of a k'Beitzah, it refers to the opinion of Rebbi Shimon. This implies that the Halachah follows the opinion of Rebbi Shimon. This is the ruling of many Rishonim (TUR OC 409).

(b) The RAMBAM disagrees with this conclusion. He writes that an Eruv must be made with a loaf equal to 6 eggs (Hilchos Eruvin 1:9). In addition, he writes that in order for a person to become Tamei with Tum'as Geviyah (when one eats food that is Tamei, one becomes Tamei with Sheni l'Tum'ah, so that if he touches Terumah he makes it Pasul), he must eat 1 1/2 eggs' worth of Tamei food (Hilchos Tum'as Ochlin 4:1). This is in accordance with the opinion of Rebbi Yochanan ben Berokah (half of 3 eggs is 1 1/2).

However, the Rambam seems to contradict himself regarding Tum'as Ochlin. The Rambam says (ibid.) that the amount of food that is subject to Tum'as Ochlin is one egg, which is in accordance with the opinion of Rebbi Shimon!

The Rambam apparently maintains that everyone agrees that the Shi'ur of Tum'as Ochlin is one egg, and that Shi'ur is not subject to the argument between Rebbi Shimon and Rebbi Yochanan ben Berokah. Their argument applies only to the Shi'ur for Tum'as Geviyah. (This also explains why Tum'as Ochlin is not mentioned in the Mishnah here.)