Eruvin 47B

שאין מן הכל זוכה אדם ללמוד. ואמר רבי יוסי: מעשה ביוסף הכהן שהלך אצל רבו לצידן ללמוד תורה.

a person does not merit to learn from everyone, and it is possible that the only suitable teacher for him lives outside of Eretz Yisrael. And R' Yosei reported in support of his position: It once happened that Yosef the priest went to his teacher in Tzeidan, outside Eretz Yisrael, to learn Torah, although the preeminent Sage of his generation, Rabban Yoḥanan ben Zakkai, lived in Eretz Yisrael.

ואמר רבי יוחנן: הלכה כרבי יוסי. ולמה לי? והא אמרת רבי יהודה ורבי יוסי ⁠— הלכה כרבי יוסי!

And R' Yoḥanan said about this: The halakha is in accordance with the opinion of R' Yosei. The Talmud asks: Why was it necessary for R' Yoḥanan to issue this ruling? Didn’t you say: In disputes between R' Yehuda and R' Yosei, the halakha is in accordance with the opinion of R' Yosei, and so it should be obvious that this halakha is in accordance with his opinion? Apparently, this principle is not accepted.

אמר אביי: איצטריך, סלקא דעתך אמינא: הני מילי ⁠— במתניתין, אבל בברייתא ⁠— אימא לא. קא משמע לן

Abaye said: It was nonetheless necessary to issue this ruling, it could have entered your mind to say that this principle applies only with regard to disputes in the Mishnah. But with regard to disputes in a baraita, say no, the principle does not apply. Therefore, R' Yoḥanan is teaching us that the halakha is in accordance with the opinion of R' Yosei in this case as well.

אלא הכי קאמר: הני כללי לאו דברי הכל נינהו, דהא רב לית ליה הני כללי.

Since no proof has been found to support Rav Mesharshiya’s statement that there are no principles for issuing halakhic rulings, the Talmud emends his statement. Rather, this is what Rav Mesharshiya is saying: These principles were not accepted by all authorities, as in fact Rav did not accept these principles, as demonstrated above.