Gemara

(R. Yehoshua ben Levi): The Halachah follows R. Yochanan ben Nuri. (One who was asleep at the start of Shabbos gets 2000 Amos in every direction.)

R. Yehoshua ben Levi also taught that the Halachah follows the lenient opinion in Eruvin.

47b: Some rams were brought [from outside the Techum]. Rava permitted people of Mechuza [who came through an Eruv] to buy them [and take them home].

Question (Ravina): Do you permit like Shmuel? R. Yochanan says that a Nochri's property acquires Shevisah, and the Halachah follows him against Shmuel!

Rava retracted.

Rishonim

Rosh (2:4): The Halachah follows the lenient opinion in Eruvin when Tana'im argue, but not when Amora'im argue. We rule like R. Yochanan against Shmuel (47b), even though Shmuel was lenient. Also, perhaps the Klal applies only to Eruvin, but not to Mechitzos. We say (95a) that perhaps the Klal that the Halachah follows R. Yehudah in Eruvin does not apply to Mechitzos. R. Chananel says that the Halachah follows R. Zeira, who is lenient about Eruvin. Maharam derived that he holds that the Halachah follows the lenient opinion in Eruvin even among Amora'im (Hagahos Maimoniyos 7 - and even regarding Mechitzos). Why do we rule like R. Yochanan against Shmuel? Since there is a Klal that the Halachah follows R. Yochanan against Shmuel, this applies even to Eruvin. The Halachah follows the lenient opinion in Eruvin when there is no Klal, like here (R. Zeira and Rabah).

Rosh (Mo'ed Katan 3:57): The Halachah follows the lenient opinion in Avelus does not apply here [regarding one who died on Shabbos], for Avelus did not yet take effect.

Hagahos Ashri (2:7): The Halachah follows the lenient opinion in Eruvin does not apply here, for there is a stringency in R. Ila'i's law. He holds that one cannot be Mevatel more than once.

Hagahos Ashri (4:9): Since we rule like R. Yochanan against Shmuel, this shows that the Halachah follows the lenient opinion in Eruvin and in Avelus only among Tana'im, but not among Amora'im. Tosfos (66a DH Yafeh) disagrees. I disagree with Tosfos.

Tosfos (48b DH Reshus): We never find that Shmuel says that the Halachah follows the lenient opinion in Eruvin. Shmuel rules like R. Yehudah against R. Shimon!

Tosfos (65b DH Ikle'u): R. Yochanan argues with R. Yehoshua ben Levi. He holds that the Kelalim [of which Tana we follow against which] apply to all of Eruvin. We derived that Rav disagrees, because in one place he ruled unlike the Kelalim. However, the Halachah follows R. Yehoshua ben Levi against R. Yochanan.

Tosfos (Mo'ed Katan 24a DH Hachi, and Kesuvos 4a DH Aval): Bahag rules like R. Yochanan. Even though the Halachah follows the lenient opinion in Avelus, this does not apply to an argument of Amora'im. The Ri says that surely the Halachah follows R. Yochanan, for the Gemara brings a support for him.

Poskim

Avkas Rochel (of the Beis Yosef, 47): The Tur and Rambam argue here. The Halachah follows the lenient opinion in Eruvin.

Ha'ara'os on Shiyurei Berachah (1): Also the Radvaz (8:50) says like Avkas Rochel.

Note: The Radvaz said "one must investigate whether the Klal applies to an argument among Poskim." He did not explicitly answer this. He gave reasons to settle the question like certain Poskim, without mentioning the Klal! In Hilchos Evel (5:9), he says that regarding Avelus, the Klal does not apply when Poskim argue.

Bach (404:1): The Rosh is stringent about whether or not Techumim apply above 10. We do not apply "the Halachah follows the lenient opinion in Eruvin", for there is no lenient opinion. All Rabanan were unsure!

Shiyurei Berachah (of Birkei Yosef, 358:2): Keneses ha'Gedolah, citing Maharimat (1:94), asked that the Rosh says that Maharam is not always lenient in Eruvin when Amora'im argue, and regarding Mechitzos. Later he brings oppositely from Maharam! Perhaps he retracted after he saw Or Zaru'a. I say that the Rosh said that Maharam ruled like the Stam Gemara, for he holds that the Klal (the Halachah follows the lenient opinion in Eruvin) applies only to Tana'im, or [perhaps the words "Eruvin, but not" must be added - PF] to Mechitzos. This shows that when we say that the Halachah follows the lenient opinion in Eruvin, it is even regarding Amora'im and Mechitzos. Maharam derived how Or Zaru'a holds, but he did not retract. This is clear from Hagahos Maimoniyos. (When he cites "my Rebbi", this is Maharam.) The Rosh applies the Klal even to Amora'im, like Or Zaru'a and R. Chananel, unlike Maharam. He cited Maharam first to show honor to his Rebbi.

Shiyurei Berachah (ibid.): Tosfos (Mo'ed Katan 24a and Kesuvos 4a) says that the Halachah follows the lenient opinion in Avelus only among Tana'im. Seemingly, Avelus is like Eruvin! This supports Ginas Veradim (YD 5:10), who says that the Ri is lenient in Avelus even among Amora'im [but Tosfos argues]. However, Maharimat was unsure if Avelus is like Eruvin. Eliyahu Rabah says that Maharam retracted, but holds that the version of the Stam Gemara overrides the Klal.

Shiyurei Berachah (3): The Tur and Shulchan Aruch rule like R. Chananel and Tosfos, that the Halachah follows the lenient opinion in Eruvin even among Amora'im! In YD 396, the Beis Yosef applies "the Halachah follows the lenient opinion in Avelus" even when Poskim argue. The same applies regarding Eruvin. The Rosh rules like Rabah, for he was Rava's Rebbi, and he is lenient in Eruvin. I say that the Tur and Shulchan Aruch held that Rava's reason is better, and this overrides the Kelalim. Also this is difficult.

Bi'ur Halachah (408 DH Rachok): Eliyahu Rabah brought a proof from Ra'avan that an Eruv more than 2000 Amos from his house does not help according to the first opinion, even though it is within the Techum. The Magen Avraham disagrees. I say that we are lenient like the Magen Avraham, for he brought a proof from the Rambam, and we are lenient in Eruvin.

Chazon Ish (Eruvin Likutim 112:10): The Halachah follows the lenient opinion in Eruvin is a special rule for Eruvin, with conditions. The Rosh (2:4) says that it is only for Tana'im, but not for Amora'im, and not for Mechitzos, even though in every case the concern is only mid'Rabanan. It seems that even if the Klal applies to Amora'im, it does not apply to Poskim, since it is not a leniency of mid'Rabanan laws. Rather, it is a Klal of the Gemara, just like "when Rav Acha and Ravina argue, the Halachah follows the lenient one." It does not apply to arguments that arose after the Gemara was sealed. The Beis Yosef decided [even laws of Eruvin] based on the majority of Poskim. He did not simply follow the lenient opinion. We do not distinguish whether a law is mid'Rabanan for one reason, or for two reasons. The Rosh discussed Mechitzos that were not Hukaf l'Dirah. Mid'Oraisa they are Mechitzos, and mid'Oraisa a valley is Mekom Petur [and he said that the Klal does not apply when Amora'im argue]. We rule like the majority of Poskim.

Beis Yosef (YD 396 b'Sof): The Halachah follows the lenient opinion in Avelus, therefore we hold like the Rosh.

Korban Nesan'el (Sof Mo'ed Katan, 40): Maharashdam (YD 201) and Mahara Sason (164) do not apply "the Halachah follows the lenient opinion in Avelus" when Poskim argue. Tosfos and the Rosh in Kesuvos do not follow the lenient opinion in Avelus when Amora'im argue, and likewise when Poskim argue.

Yabi'a Omer (1 YD 25): It is well known that the Halachah follows the lenient opinion in Avelus even when Poskim argue (Shiyurei Keneses ha'Gedolah, Ne'eman Shmuel, Beis David). The Chida, Shulchan Gavoha, Degel Machaneh Efrayim and others do not follow the lenient opinion in Avelus against the Shulchan Aruch. In addition, many hold that we do not follow the lenient opinion when Poskim argue (Korban Nesan'el, Radvaz, Zera Emes).

Tzitz Eliezer (11:81, citing Beis David YD 194): "The Halachah follows the lenient opinion in Avelus" does not apply to something that all the Poskim rejected.