Bekhorot 24A

הלכה כרבי יוסי בן המשולם למה לי?

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

אי אמר הלכתא דכוליה פירקין, ולא אמר הלכתא כרבי יוסי בן המשולם, הוה אמינא: אדרבי יוסי בן המשולם קאי, ומאי ״בכוליה פירקין״? דרבי יוסי תרתי אמר, ופלוגתא דברייתא פלוגתא היא.

The Talmud 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 R' Yosei ben HaMeshullam, I would say: Rav is referring specifically to the opinion of R' 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 R' Yosei said two separate statements in the chapter. And this would indicate with regard to Rav’s statement that if a Mishnah is contradicted by a dissenting opinion cited in a baraita, that is considered a dispute.

אשמעינן הלכה כרבי יוסי ⁠— לאשמועינן דאדרבן שמעון בן גמליאל קאי, ופלוגתא דברייתא לאו פלוגתא היא.

Rav taught that the halakha is in accordance with the opinion of R' 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 Mishnah is contradicted by a dissenting opinion cited in a baraita, that is not considered a dispute.

מאי ברייתא? דתניא: הלוקח בהמה מניקה מן הגוי, הבא אחריו בכור מספק, מפני שמרחמת אף על פי שאינה יולדת. רבן שמעון בן גמליאל אומר: דבר בחזקתו.

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 non-Jew, 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.