why do I need him to state that the halakha is in accordance with the opinion of Rabbi Yosei ben HaMeshullam?

The Gemara explains: Had Rav said only that the halakha is in accordance with the opinion of the anonymous tanna in the mishnayot throughout the whole chapter, but did not say explicitly that the halakha is in accordance with the opinion of Rabbi Yosei ben HaMeshullam, I would say: Rav is referring specifically to the opinion of Rabbi Yosei ben HaMeshullam. And what is the meaning of the phrase: Throughout the whole chapter, which indicates he is speaking of more than one case? This is referring to the fact that Rabbi Yosei said two separate statements in the chapter. And this would indicate with regard to Rav’s statement that if a mishna is contradicted by a dissenting opinion cited in a baraita, that is considered a dispute.

Rav taught that the halakha is in accordance with the opinion of Rabbi Yosei in order to teach us that his statement with regard to the whole chapter is referring to the opinion of Rabban Shimon ben Gamliel, and with regard to Rav’s statement, if a mishna is contradicted by a dissenting opinion cited in a baraita, that is not considered a dispute.

What is the baraita mentioned previously? As it is taught in a baraita: In the case of one who purchases a nursing female animal from a gentile, the young that is born after the offspring currently being nursed is treated as a firstborn due to the uncertainty whether it is the animal’s offspring, because an animal might be merciful and nurse another animal even though it has not given birth at all. Rabban Shimon ben Gamliel says: The matter stands in its presumptive status, i.e., the nursing animal is assumed to be the mother of the nursing offspring.

The baraita continues: And similarly, Rabban Shimon ben Gamliel would say: One who enters amid his flock at night and sees about ten or about fifteen animals, some of whose offspring is a firstborn and others whose offspring is not a firstborn, and all were giving birth, and on the next day he arose and found that the male young were nursing from the mothers whose offspring is not a firstborn, and the female young were nursing from the mothers whose offspring is a firstborn, he does not need to be concerned that perhaps the young of this mother came to nurse from that mother. Rather, he may assume that each animal is nursing its own offspring.

A dilemma was raised before the Sages: What is the precise meaning of the phrase: The matter stands in its presumptive status, which Rabban Shimon ben Gamliel stated? Does this mean that an animal does not show mercy unless it has given birth, and consequently, in a case where the animal has given birth we are concerned for the possibility that it might be merciful and nurse an offspring that is not its own? Or perhaps it means that an animal might be merciful only toward its own offspring, but it does not show mercy to an animal that is not its own?

The Gemara asks: What difference is there in practice based on this distinction? Either way the nursing animal is assumed to have given birth and its future offspring will not be considered a firstborn. The Gemara answers that there is a difference with regard to whether or not one is flogged for slaughtering it due to violating the prohibition against slaughtering a mother and its offspring on the same day. If you say an animal is merciful only toward its own offspring but is not merciful toward an animal that is not its own, then one is flogged if he deliberately slaughters an animal and the offspring that it is nursing. But if you say it is also merciful toward an animal that is not its own, then one is not flogged in such a case.

The Gemara returns to its initial inquiry. What is the precise meaning of Rabban Shimon ben Gamliel’s statement? Come and hear a proof from the mishna, as Rabban Shimon ben Gamliel says: In the case of one who purchases a nursing female animal from a gentile, he does not need to be concerned lest the offspring currently being nursed was the offspring of another animal. Evidently, Rabban Shimon ben Gamliel holds that an animal nurses only its own offspring. The Gemara rejects this proof: Does the mishna teach that one does not need to be concerned that perhaps it is the offspring of another animal? No, it teaches that one does not need to be concerned that perhaps it was the offspring of another animal, and this is what it is saying: Whichever way you look at it, the next offspring is not a firstborn, as one does not need to be concerned that perhaps it was the offspring of another animal that it is nursing, as it would not nurse unless it already had an offspring initially.

The Gemara suggests: Come and hear a proof from the continuation of the mishna: In the case of one who enters amid his flock and sees mother animals that gave birth for the first time that were nursing, and also sees mother animals that gave birth not for the first time that were also nursing, he does not need to be concerned that perhaps the offspring of this animal came to that animal to be nursed, or that perhaps the offspring of that animal came to this animal to be nursed.

The Gemara explains the proof: But why not? Let us be concerned that perhaps the nursing animal is merciful toward an offspring that is not its own. Evidently, Rabban Shimon ben Gamliel holds that an animal nurses only its own offspring, even if it had previously given birth. The Gemara rejects this proof: Even if an animal does show mercy toward offspring that is not its own, in a place where it has its own offspring, it does not set aside its own offspring and show mercy toward offspring that is not its own.

The Gemara further suggests: Come and hear a proof from Rabban Shimon ben Gamliel’s statement in the baraita: If one purchases a nursing animal from a gentile, the matter stands in its presumptive status. From the phrase: And similarly, in the continuation of the baraita, the Gemara infers: What, is it not so that the first clause is similar to the latter clause? Just as in the latter clause the nursing young is certainly considered to be the nursing animal’s own offspring, so too, in the first clause it is certainly considered to be the nursing animal’s own offspring, despite the fact that the animal mother might have given birth previously.

The Gemara rejects this suggestion: Are the cases comparable? This first clause of the baraita stands as it is and that second clause of the baraita stands as it is, i.e., they are referring to different cases. In the second clause, where all the offspring are standing before their mothers, it is certain that each mother will nurse only its own child. Conversely, in the first clause it is uncertain whether that child is its own. But if so, what is the meaning of the phrase: And similarly? It means both clauses state that the nursing animal is exempt from having its future offspring counted a firstborn, since even if that offspring is not the mother’s, the mother would not nurse it unless it had previously given birth.

The Gemara analyzes a similar case. Rabba bar bar Ḥana says that Rabbi Yoḥanan says: If one saw a young animal resembling a pig that was trailing after a ewe, the ewe is exempt from having its future offspring counted a firstborn. And the pig-like animal is forbidden for consumption “until he comes and instructs righteousness to you” (Hosea 10:12), i.e., until Elijah the prophet arrives and determines the halakha.

The Gemara raises a difficulty with regard to Rabba bar bar Ḥana’s statement: On the one hand, the ewe is exempt from having its future offspring counted a firstborn, which is in accordance with the opinion of Rabban Shimon ben Gamliel that an animal that nurses from another is presumed to be its offspring. But on the other hand, the pig-like animal is forbidden for consumption, which is in accordance with the opinion of the Rabbis that an animal might nurse the young of another.

And furthermore, if the halakha is in accordance with the opinion of the Rabbis, and there is a concern with regard to the identity of the mother of the offspring, what is the reason for citing the verse: “Until he comes and instructs righteousness to you”? By citing it, the baraita is indicating that the status of the pig-like animal is a halakhic uncertainty that can be resolved only by Elijah the prophet. Instead, it should have stated: Until the matter becomes known to you, in reference to the empirical uncertainty as to whether the pig-like animal is the offspring of the ewe.

And if you would say that it is uncertain whether the halakha is in accordance with the opinion of Rabban Shimon ben Gamliel or in accordance with the opinion of the Rabbis, that cannot be correct: If that matter is uncertain, then why is the ewe exempt from having its future offspring counted a firstborn? And furthermore, is this question of whose opinion is accepted as the halakha an uncertainty at all? But doesn’t Rabba bar bar Ḥana say that Rabbi Yoḥanan says: Wherever Rabban Shimon ben Gamliel taught a halakha in the corpus of our Mishna the halakha is in accordance with his opinion, except for the following three cases: The responsibility of the guarantor (see Bava Batra 174a), and the incident that occurred in the city of Tzaidan (see Gittin 74a), and the dispute with regard to evidence in the final disagreement (see Sanhedrin 31a)?

The Gemara answers: Actually, it is obvious to Rabba bar bar Ḥana that the halakha is in accordance with the opinion of Rabban Shimon ben Gamliel. But it is uncertain if Rabban Shimon ben Gamliel holds that an animal that has given birth might be merciful toward offspring that is not its own, or if he holds that it is not merciful toward the offspring of another. The Gemara rejects this claim: If that is so, instead of teaching a case involving an animal with the appearance of a pig, he should have taught the more common case of a mother ewe nursing a baby lamb. And accordingly, rather than inquiring with regard to the prohibition against eating a non-kosher animal, he should have inquired as to whether or not one is flogged for it due to violating the prohibition against slaughtering a mother and its offspring on the same day.

The Gemara explains: It was necessary for Rabba bar bar Ḥana to teach the case of a pig, as, had he taught a case involving a lamb, I would say: Even if you say that Rabban Shimon ben Gamliel holds that an animal that has given birth might be merciful toward the offspring of another, this is so only with regard to its own species, but with regard to an animal that is not of its own species, it would not show mercy and would not nurse the offspring. Therefore, Rabba bar bar Ḥana teaches us the case of a pig, to convey that an animal might be merciful, and this is also true even where the offspring is not of its own species, as it could be said that perhaps the animal was merciful in such a case as well.

And this is the meaning of that which Rabba bar bar Ḥana said that Rabbi Yoḥanan said: A dilemma was raised