1) BREAKING DOWN A WOODEN "MIGDAL" ON SHABBOS
QUESTION: The Mishnah (34b) teaches that when an Eruv is placed in a Migdal (a large cupboard) and then the key to the Migdal is lost, the Eruv nevertheless is valid according to the Tana Kama. Rebbi Eliezer says that the Eruv is not valid unless the person knows that the key is in its normal place.
Rabah and Rav Yosef explain that the Migdal mentioned in the Mishnah is made of wood. Therefore, the dispute between the Tana Kama and Rebbi Eliezer is based on whether the Migdal is considered a utensil ("Kli") or a building ("Ohel"). If it is a Kli, then one may break it (or break the rope which keeps it closed) on Shabbos in order to get the food of the Eruv. If it is an Ohel, one may not break it on Shabbos (because of the Melachah of "Soser Binyan").
The Gemara says that we find the same dispute in the Mishnah in Zavim (4:3), which discusses a case of a Zav who knocks on a large vessel. According to the Tana Kama, the Zav's knock causes the vessel to become Tamei (through Tum'as Heset), presumably because the vessel has the status of a Kli. According to Rebbi Nechemyah and Rebbi Shimon, the vessel remains Tahor, presumably because the vessel has the status of an Ohel (which cannot become Tamei through Heset).
Rabah and Rav Yosef's statement is difficult to understand. If the Migdal has a capacity of forty Se'ah, then it is an Ohel. If it does not hold forty Se'ah, then it is a Kli. How can the Tana'im in the Mishnah here, and the Tana'im in the Mishnah in Zavim, argue whether the Migdal is considered an Ohel or a Kli? Its status depends upon its size!
(a) TOSFOS RABEINU PERETZ explains that the Mishnah refers to a Migdal that holds forty Se'ah. However, since it is often moved around even when full, it might have the status of a Kli and not the status of an Ohel. This is the subject of dispute between the Tana Kama and Rebbi Eliezer in the Mishnah here, and between the Tana'im in the Mishnah in Zavim.
(b) The TOSFOS HA'ROSH and the RITVA also explain that the Mishnah refers to a vessel that holds forty Se'ah. However, they explain the dispute among the Tana'im differently.
In the Mishnah in Zavim, the Tana'im do not argue about the status of the vessel, but rather whether the contents (such as food or utensils) inside the vessel become Tamei or not. According to one Tana, the movement of the contents of the vessel is directly caused by the Zav's knock on the vessel (because the vessel is light enough that the movement of its contents is directly related to the Zav's knock). According to the other Tana, since the vessel is so heavy, the movement of the contents is a secondary result of the Zav's knock, and thus his knock does not make them Tamei through Heset (by causing them to move).
The same argument exists with regard to whether the Melachah of Soser Binyan applies to a Migdal or not. According to the Tana Kama, since a large Migdal that holds more than forty Se'ah is light enough that it moves when one knocks on it, it is not considered a building with regard to destruction on Shabbos (even though it is considered an Ohel in other respects). According to Rebbi Eliezer, however, the Migdal is heavy enough to have the status of a building, and therefore the Melachah of Soser Binyan does apply.
2) A "CHAZAKAH" DURING "BEIN HA'SHEMASHOS"
QUESTION: The Mishnah (35a) discusses a case of an Eruv made with food of Terumah that became Tamei. Rebbi Meir and Rebbi Yosi disagree whether the Eruv is valid or not when it is unclear if the food became Tamei before nightfall (in which case the Eruv is invalid) or after nightfall (and the Eruv is valid).
Rebbi Meir maintains that an Eruv in doubt is not a valid Eruv. The Gemara asks how Rebbi Meir can be stringent in a case of doubt, when elsewhere (Mikva'os 2:1) he is lenient in a case of doubt. Rebbi Yirmeyah answers that in the Mishnah here it is known for certain that the Terumah became Tamei at the beginning of Bein ha'Shemashos (that is, a Sheretz fell on it then).
The Gemara rejects this answer because if it is known that the Terumah became Tamei at the beginning of Bein ha'Shemashos, then why does Rebbi Yosi say that the Eruv is valid?
REBBI AKIVA EIGER (in Gilyon ha'Shas) asks that the Gemara should have given a simple answer to explain the dispute in the Mishnah. TOSFOS in Shabbos (34a, DH Sheneihem) explains that a Chezkas Taharah is effective only when one is not sure when the object became Tamei (for example, before or after Bein ha'Shemashos). If, however, it is known for certain that the object became Tamei during Bein ha'Shemashos, but it is unclear whether Bein ha'Shemashos is day or night, one may not use the Chazakah to determine that the object was Tahor until nightfall. A Chazakah tells us only that the object retains its status quo until the latest possible moment (that is, until the moment at which there is no doubt about its status). In the case of an object that became Tamei during Bein ha'Shemashos, it is known exactly when the item became Tamei. The only doubt is whether that point in time was considered day or night. The Chazakah cannot clarify that Bein ha'Shemashos is considered day and that the food is Tahor.
Accordingly, the Gemara should have answered that Rebbi Meir maintains that the food of the Eruv is Tamei in the case of the Mishnah, because it discusses a case in which it is known that a Sheretz fell onto it during Bein ha'Shemashos. A Chazakah cannot determine if the moment during Bein ha'Shemashos at which the Sheretz fell on the Terumah is day or night. Rebbi Yosi, however, maintains that it is a valid Eruv, because he maintains that any Eruv in doubt is valid. Why does the Gemara not suggest such a case to explain the argument in the Mishnah?
(a) RASHI in Shabbos (34a) seems to disagree with the assertion of Tosfos there that a Chezkas Taharah is effective only when one is not sure when the object became Tamei. Rashi maintains that a Chazakah applies even when it is known for certain that the object became Tamei during Bein ha'Shemashos. Even though one knows exactly when the Sheretz fell on the Terumah, the Chazakah tells us that that moment during Bein ha'Shemashos was Halachically nighttime (and thus the Eruv is valid). This is because we know that until now, throughout the entire day before Bein ha'Shemashos, the Terumah was Tahor. The Chazakah allows us to assume that the point in time (during Bein ha'Shemashos) at which it became Tamei was nighttime.
Therefore, according to Rashi, the Gemara does not answer that this is the case in which Rebbi Meir and Rebbi Yosi argue, because in such a case Rebbi Meir agrees that it is a valid Eruv because of the Chazakah.
(b) The RASHASH points out that the Gemara in Berachos (2a) implies that Rebbi Meir and Rebbi Yosi both maintain that the duration of Bein ha'Shemashos is not more than "k'Heref Ayin," the blink of an eye. Accordingly, there can be no doubt whether the food became Tamei during daytime or nighttime when the Sheretz fell on it during Bein ha'Shemashos. Since Bein ha'Shemashos is the length of the blink of an eye, the Sheretz cannot fall exactly at that moment. The doubt must be that it is unclear whether the Sheretz fell on the food during daytime (before Bein ha'Shemashos) or after nightfall (after Bein ha'Shemashos).
(c) Perhaps when Rebbi Yirmeyah says that there was a Sheretz on the Terumah at the beginning of Bein ha'Shemashos, he means to say exactly what Rebbi Akiva Eiger asks that the Gemara should say. Moreover, the Gemara here is probably the source for Tosfos' suggestion in Shabbos that a Chazakah does not apply when it is known that the Sheretz fell on the Terumah during Bein ha'Shemashos. Tosfos understands that Rebbi Yirmeyah suggests that the argument in the Mishnah involves Rebbi Akiva Eiger's case, where the Sheretz fell on the Terumah at a known point during Bein ha'Shemashos.
If this is Rebbi Yirmeyah's intention, then why does the Gemara ask that Rebbi Yosi should agree that the Eruv is not valid? Since a Chazakah does not apply in this case, the doubt (whether the moment at which the Sheretz touched the Terumah is considered day or night) remains, and Rebbi Yosi rules that an Eruv in doubt is valid.
The RASHASH explains that Rebbi Yosi indeed would not permit a doubtful Eruv in such a case. The Gemara later (36a) concludes that according to Rebbi Yosi, an Eruv in doubt is valid only because of a Chazakah. (This is also clear from Tosfos in Shabbos.) Without a Chazakah, Rebbi Yosi would not permit use of the Eruv. Therefore, the case of the Mishnah cannot be one in which a Sheretz fell on the Terumah at a known point during Bein ha'Shemashos.