Gemara

If a Karfef more than Beis Se'atayim which was not Hukaf l'Dirah, one may carry only within four Amos.

25a - Version #1: A pillar 10 tall and four wide is Mema'et (diminishes) a Karfef that is Beis Sa'atayim. If it is less than three wide, it is not Mema'et.

(Rabah): A pillar between three and four wide is Mema'et;

(Rava): It is not Mema'et.

(Rabah): A Mechitzah [l'Shem Dirah] between three and four Tefachim away from the wall of the Karfef, it helps.

(Rava): It does not help.

Version #2 (Rav Simi): Rabah and Rava agree that a pillar three wide is Mema'et. They argue about a pillar less than three.

(Rabah): If one plastered the wall with mud, and it cannot stay by itself [without the wall], it is Mema'et;

(Rava): It is not Mema'et.

46a (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.

(R. Zeira): He needed to teach the Klal (general rule), and also to rule like R. Yochanan ben Nuri, to teach that we follow the lenient opinion in Eruvin, even when an individual argues with a Rabim.

Question (Rava): Eruvin is mid'Rabanan. Why would we distinguish whether an individual argues with an individual, or with a Rabim?

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

Rambam (Hilchos Shabbos 16:8): If one built a pillar next to the wall at least 10 tall and three wide, it is Mema'et. If it is less than three wide, it is not Mema'et.

Rambam (9): If one plastered over the old Mechitzos in order to diminish the airspace, this helps only even if it cannot stand by itself. If one distanced three Tefachim from the mound and built a Mechitzah, it helps.

Rosh (2:4): Maharam rules unlike Rav Simi, rather, like Version #1, which is the Stam Gemara. 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.

Rosh (ibid.): R. Chananel says that the Halachah follows R. Zeira (Or Zaru'a rules like Rabah), who is lenient about Eruvin. Maharam learned from him 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 (5:3): Maharam says that the Halachah follows R. Meir, since R. Yehoshua ben Levi said that the Halachah follows the lenient opinion in Eruvin, even when an individual argues with a Rabim.

Rosh (6:9) and Tosfos (66a DH Yafeh): R. Chananel rules like R. Yochanan, for the Halachah always follows him against Shmuel, and like the lenient opinion in Eruvin. The Ri rules like Shmuel, for a Beraisa (70b) supports him.

Rosh (ibid.): Maharam rejected the proof, and ruled like R. Yochanan for R. Chananel's reasons.

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 (46a DH d'Amar): It seems that we rule like the lenient opinion in Eruvin even if it is mid'Oraisa. The Gemara said 'Eruvin is mid'Rabanan. Why does it matter if an individual argues with a Rabim?' This implies that if it were mid'Oraisa, [we would still be lenient, just] it would be a Chidush.

Mordechai (499): The Halachah follows R. Yochanan ben Nuri, for the Halachah follows the lenient opinion in Eruvin. Regarding Hefker, Chachamim are lenient, and a Nochri's property acquires Shevisah, like R. Yochanan. Maharam derived that the Halachah follows the lenient opinion in Eruvin only among Tana'im, but not among Amora'im. Bahag says so about Avelus. He rules like R. Yochanan that private matters apply on Shabbos, even though Rav and Shmuel argue with him. However, the Ri (in Tosfos 66a) connotes that the Klal applies even to Amora'im. Maharam disagreed.

Poskim

Shulchan Aruch (OC 358:5): If one built a pillar at least three wide, it is Mema'et.

Beis Yosef (DH Banah): The Rosh says that Maharam rules like Version #1. The Rambam agrees. Maharam, the Ra'avad and Ba'al ha'Ma'or rule like Rava. The Rambam rules like Rabah. I agree, for the Halachah does not follow a Talmid against his Rebbi.

Gra (DH Banah): In Sa'ifim 5-7, the Tur and Shulchan Aruch rule like Rava against Rabah, and like Rav Simi, who is lenient. The Rosh (2:4) brought from Maharam that the Halachah does not follow Rav Simi. Even though the Halachah follows the lenient opinion in Eruvin, this refers to [arguments among] Tana'im, but not to Amora'im. Also, [perhaps the Klal does not apply to Mechitzos]. The Rosh wrote that later, Maharam retracted from both reasons, and rules like Rava. The Rosh held like Rabah, i.e. initially, but he concluded like Rav Simi and unlike Rabah. The Rambam (16:9) wrote like Version #1, for he does not follow the lenient opinion among Amora'im, like the Mordechai, Hagahos Maimoniyos (7) and Hagahos Ashri (4:9 DH mi'Kan) say. They proved this from 47b, in which we rule like R. Yochanan.

Gra (ibid.): The Rosh answered Maharam's question (why we rule stringently like R. Yochanan) that the rule (the Halachah follows the lenient opinion in Eruvin) does not apply against a Klal (general rule, e.g. the Halachah follows R. Yochanan against Shmuel). This is difficult, for the Rosh (5:3) brought from Maharam that the Halachah follows the lenient opinion in Eruvin, even an individual against a Rabim. The greatest Klal there is, is that the Halachah follows the Rabim! The Mordechai says that due to this, in Mo'ed Katan (24a), Bahag and Tosfos (DH Hachi) rule like R. Yochanan regarding Avelus against Rav and Shmuel, even though [the Halachah follows the lenient opinion in Avelus], unlike the Rosh's answer. Tosfos (46a) says that the Halachah follows the lenient opinion in Avelus, even an individual against a Rabim! However, we can answer like Tosfos in Kesuvos (4a, that the Halachah follows R. Yochanan, for the Gemara supports him). The Rosh (Mo'ed Katan 3:57) said similarly. However, Tosfos contradicts what he wrote (66a DH Yafeh, that we the follow the lenient opinion in Eruvin, even regarding Amora'im). Also the Rosh says so there (6:9). The Mordechai proved from this Tosfos. Hagahos Ashri derived this from Maharam, but he disagreed, for 47b teaches that it does not apply to Amora'im.

Shulchan Aruch (6): If one built a Mechitzah 10 long in front of the old Mechitzah, so it will be Hukaf l'Dirah via the new Mechitzah, if he distanced three Tefachim from the old Mechitzah, it is permitted.

Kaf ha'Chayim (59): One must intend that the Mechitzah be permanent.

Shulchan Aruch (7): If one plastered over the old Mechitzos in order to diminish the airspace, this helps only if it is thick enough that it could stand by itself.

Birkei Yosef (Shiyurei Berachah 3): Why are the Tur and Shulchan Aruch stringent? They did not even mention that the Rosh and Rambam are lenient! The Tur almost always follows the Rosh, and whenever two of the Rif, Rambam and Rosh agree, the Shulchan Aruch follows them! Also, 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. Shiyurei Keneses ha'Gedolah says "the Prishah asked this. He said that do not say that they follow Rabim (Maharam, Ba'al ha'Ma'or, and the Ra'avad), who rule like Rava. If so, they should rule like him also regarding a Mechitzah and mound!" It is not clear to me that Maharam, Ba'al ha'Ma'or, and the Ra'avad rule like Rava regarding all of these. Perhaps they argue only here, and agree about the others. The Rosh connotes like this. He mentioned that they argue only regarding plastering. This is difficult, for above the Rosh and Hagahos Maimoniyos said that Maharam rules like the first version, since the Klal does not apply Amora'im! The Ba'al ha'Ma'or explicitly is stringent like Rava in all three laws! 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.

Mishnah Berurah (55): Some permit even if it can stand only with the wall. One may be lenient when necessary.