Question: This is difficult for Shmuel, who taught below (61b) that if one spent Shabbos in a desolate city, according to Rabanan he may walk in all of it and 2000 Amos outside. If he put his Eruv in a desolate city, he has only 2000 Amos from its place! (This is a big stringency.)

Answer: In any case, in a normal case, when he does not put his Eruv in a desolate city, it is a leniency.

Explanation (R. Tam): [All agree that he must say] that it is hot now or cold now. Since he returned him due to this reason, if so he intends to go when the heat or cold will pass, and he does not lose the status of an Oni;

However, if he returned him without a reason, if so he totally despaired from going, and he lost the status of an Oni, even though he started going.

Rav Yosef says that all agree that he must start going. They argue about whether or not he must say;

Rav Yosef holds that since he returns due to heat or cold, if so he totally despaired from going. The Eruv takes effect according to R. Yehudah only when he returns without a reason;

R. Yosi said a bigger Chidush. Even so, the Eruv takes effect.

Implied question: Rav Yosef should have said that they argue b'Omer (in a case that he says)!

Answer: Rav Yosef said Lomar (which connotes that they argue about whether or not he must say) for parallel structure to Rabah;

Rav Yosef deduced (that they argue about whether or not he must return due to a reason) since R. Yehudah did not mention that it is hot now or cold now, and R. Yosi mentioned this.

Explanation #1: [Ula said that] even though he is returned, he is Muchzak. Like whom is this? It is like Rav Yosef, according to R. Yosi.

[The Gemara] infers from the words "even though" that he returned due to heat or cold, and his Eruv is not valid according to R. Yehudah.

It is not like Rabah, for Rabah holds that all agree that he must say "it is cold now." If he said "it is cold now", "even though" is not appropriate.

Rebuttal (and Explanation #2): This Perush is difficult. The Ri says that Rashi's latter Perush is primary. (Rabah says that all agree that he must say, i.e. declare his Shevisah. They argue about whether or not he needs Hichzik (he started walking). Rav Yosef holds that all require Hichzik. They argue about whether or not he must say. Ula does not require saying. This is like Rav Yosef, according to R. Yosi.)

Explanation #1 (Rashi): [The Gemara] infers from "even though", which connotes that he is returned from acquiring Shevisah. If so, he did not say.

Explanation #2: It infers that if he required both Hichzik and saying, if so he should say that the Halachah follows R. Yehudah, for according to both Rav Yosef and Rabah, R. Yehudah requires both.

Explanation #1: R. Meir considers every Safek to be Chamar Gamal. Also our Mishnah is a Safek.

Question: How can we learn from there (a Safek when the Eruv became disqualified) that here it is a Safek according to R. Meir?

Explanation #2 (Ri): He asks that he should have taught "anyone who could have [been Me'arev, and was not Me'arev], it is a Safek", and automatically (from what R. Meir taught above) we would know that it is Chamar Gamal!

[Rav Sheshes] answers that when Vadai he was not Me'arev, like here, it is not Chamar Gamal. Even had it taught Safek, we would have explained that it is a Safek whether or not he is considered an Oni, like others on the road, and therefore his Eruv is invalid;

They said that a Vadai Oni can be Me'arev with his feet "in place Ploni", but not a Safek Oni. His law is like a Vadai Ashir, and he is like the people of his city. The Mishnah teaches that this is not so. (It is a Safek, and it is Chamar Gamal.)

Explanation: Even if his measure finished in a cave, he may not enter one Amah more. Rashi's Perush (if one declared his Shevisah to be in his place, and measured his 2000 Amos and they ended even one Amah outside the Techum of a city, he may not enter, for the 2000 was in addition to his four Amos) is not correct. (Tosfos ha'Rosh - even if his measure ended in the city, he may not walk past it.)

Implied question: Why didn't it say that the Beraisa teaches that one who was one Amah outside the Techum when Shabbos came, he may not enter?

Answer: Obviously, he may not go outside his Techum! However, it needs to teach one whose measure finished in a cave.

Explanation (R. Chananel): Pagum Nichnas is a building made like a dovecote, like it says in Sanhedrin (25b) regarding bird racers (gamblers. Their Teshuvah to become Kosher for testimony) is from when they break Pigmeihem;

However, an actual dovecote does not get added to the city, like we say in the Gemara.