WHY IS REBBI YOSI MACHSHIR?
Answer #2 (Rava): There, R. Meir is lenient because two Chazakos support this. (Shimon was Muchzak to be alive, and Reuven was Muchzak to be Tahor.) Here, he is stringent because there is only one Chazakah to be lenient (the Terumah was Tahor. Texts that say 'one Chazakah to be stringent' refer to Chezkas Isur to leave the Techum of his city [without a valid Eruv].)
Question: R. Yosi contradicts himself! (He is lenient in our Mishnah, but he is stringent about Safek immersion in the Mishnah cited on 35b.)
Answer (Rav Huna bar Chinena): He is more stringent about Tum'ah, for it has a basis in Torah.
Question: Also [Techumim of] Shabbos has a basis in Torah [according to R. Akiva]!
Answer #1: He holds that Techumim is mid'Rabanan.
Answer #2: He himself is stringent. In our Mishnah he gives the opinion of his Rebbi.
Support (Mishnah - R. Yosi): Avtulmus testified in the name of five Chachamim that a Safek Eruv is valid.
Answer #3 (Rava): Regarding the Mikveh, R. Yosi holds that we leave the Tamei person on his Chazakah. We assume that he did not immerse. (Regarding Eruv, the Chazakos of the Terumah (it is Tahor) and his Techum oppose each other. Since it is mid'Rabanan, we are lenient.)
Question: Just the contrary, we should leave the Mikveh on its Chazakah, and assume that it is still full!
Answer: The case is, we never measured the Mikveh. (Perhaps it was never full.)
(Beraisa - R. Yosi): A Safek Eruv is valid in the following cases:
One was Me'arev with Terumah, and we do not know whether it became Tamei before Shabbos or after dark;
He was Me'arev with [Tevel] produce, and we do not know whether it was fixed (tithed) before Shabbos or after dark.
The following Safek Eruvin are invalid --
He was Me'arev with Terumah, and we do not know whether it was Tamei or Tahor from the beginning;
He was Me'arev with produce, and we do not know whether or not it was fixed.
Question: He is Machshir regarding Terumah, because we leave it on its Chazakah. (We assume that it was Tahor all of Bein ha'Shemashos.) Likewise, we should leave Tevel on its Chazakah [and assume that it was Tevel all of Bein ha'Shemashos]!
Correction: That clause should say 'if one was Me'arev with produce, and we do not know whether it became Dimu'a (mixed with Tevel) before Shabbos or after dark.' (For all of Bein ha'Shemashos, the Chazakah is that it was not Dimu'a. We do not explain that it was standard Dimu'a, i.e. mixed with Terumah, because our Tana permits an Eruv of Terumah.)
AN ERUV MUST BE PERMITTED BEFORE SHABBOS
Question (Rav Shmuel bar Rav Yitzchak): If one had a Tahor loaf and a Tamei [Terumah] loaf, and he said [to a Shali'ach] 'be Me'arev with the Tahor loaf, wherever it is' [and the Shali'ach placed both for the Eruv], what is the law? (Ritva - the same applies if he did so himself. We discuss a Shali'ach, for this is the typical case. Most who Me'arev themselves do so through walking, without bread.)
We ask according to R. Meir and according to R. Yosi.
R. Meir is Posel when we do not know that the loaf was Tahor all of Bein ha'Shemashos. Here, since we know that it was Tahor, perhaps he would be Machshir!
R. Yosi is Machshir when the Eruv remained Tahor all of Bein ha'Shemashos, for we know which loaf it is [and the Chazakah is that it remained Tahor]. Here, since we do not know which loaf is Tahor, perhaps he would be Posel!
Answer (Rav Huna): Both of them Posel. We require an Eruv that one could have eaten before Shabbos, and this is lacking. (No one may eat either loaf, for each is Safek Tamei Terumah.)
Question (Rava): If one said [on Erev Shabbos] 'this loaf is Chulin today and Hekdesh tomorrow' and was Me'arev with it, is it valid?
Answer (Rav Nachman): It is valid.
Question (Rava): If one said 'It is Hekdesh today and Chulin (redeemed onto particular coins) tomorrow' and was Me'arev with it, is it valid?
Answer (Rav Nachman): It is invalid.
Question (Rava): What is the difference between these two cases?
Answer (Rav Nachman): In both cases, Bein ha'Shemashos we are in doubt about its status (Chulin or Hekdesh, for we do not know if it is day or night). Therefore, we leave it on its Chazakah from before Shabbos.
(Mishnah): If a Tevul Yom flask (it was immersed today) was filled with Ma'aser that is Tevel [to Terumas Ma'aser], and the owner said 'it should be Terumas Ma'aser at dark', this takes effect;
If he was Me'arev with it, it is invalid.
(Rava): This teaches that an Eruv acquires Shevisah at the end of the day (Bein ha'Shemashos, the end of Erev Shabbos. It is still Tevel then);
Proof: If the beginning of the day (Shabbos) acquired, the Eruv would be valid! (The Ma'aser is then Terumas Ma'aser. It is permitted to a Kohen.)
Rejection (Rav Papa): Perhaps indeed, the beginning of the day acquires. We require an Eruv permitted before Shabbos, and here it was not.
AN ERUV THAT DEPENDS ON BREIRAH
(Mishnah): One may stipulate about an Eruv and say 'if Nochrim come from the east, my Eruv is in the west (I will flee to there). If they come from the west, my Eruv is in the east. If they come from both directions, I will go where I want. (Retroactively, my Eruv was in that direction.) If they do not come, I am like people of my city (without an Eruv).'
One may say 'if a Chacham comes from the east, my Eruv is in the east. If a Chacham comes from the west, my Eruv is in the west. If Chachamim come from both directions, I will go where I want. If no Chacham comes, I am like people of my city.'
R. Yehudah says, if one of the Chachamim is his Rebbi, he may go in the direction of his Rebbi. If both are his Rebbi'im, he may go in whichever direction he wants.
(Gemara) Question: R. Yitzchak taught a Beraisa just the opposite of our Mishnah! (He stipulates that his Eruv be in the direction of the Nochrim, and the opposite direction from the Chacham);
This contradicts our Mishnah regarding both Nochrim and a Chacham!
Answer - part 1: Our Mishnah discusses a Nochri tax collector. The Beraisa discusses the mayor. (One needs to appease him or make an appeal to him.)
Answer - part 2: The Mishnah discusses a Chacham who teaches, and the Beraisa discusses [when a Chacham comes to teach in one direction, and] one who trains children to pray comes in the other direction. (He may go in the opposite direction from the children's teacher, which is the direction of the Chacham.)
(Mishnah - R. Yehudah): If one of the Chachamim is his Rebbi, he goes in the direction of his Rebbi.
Question: Why do Chachamim disagree? (Surely, he intended to go to him from before Shabbos!)
Answer: Sometimes a person prefers his friend over his Rebbi.
(Rav): We learn from the Beraisa of Ayo (a Chacham) that our Mishnah is mistaken!
(Ayo - Beraisa - R. Yehudah): A person cannot stipulate about two things simultaneously;
He can say 'if a Chacham comes from the east, my Eruv is in the east. If he comes from the west, it is in the west'; but he cannot stipulate 'if Chachamim come from both directions, I will go where I want.'
Question: Surely, he cannot stipulate both because [R. Yehudah holds that] Ein Breirah. For the same reason, he should not be able to stipulate about when a Chacham comes from one direction!
Answer (R. Yochanan): The case is, the Chacham already came [before Bein ha'Shemashos, but the Me'arev did not know. It is already determined, so it does not depend on Breirah].