The Gemara answers: The mishna is incomplete and it teaches the following: If one placed the eiruv in a cupboard and locked it, and the key is lost, it is nonetheless a valid eiruv. In what case is this statement said? On a Festival; however, on Shabbat, his eiruv is not a valid eiruv. If the key is found, whether in a city or in a field, his eiruv is not a valid eiruv. Rabbi Eliezer disagrees and says: If it is found in a city, his eiruv is a valid eiruv; but if it is found in a field, his eiruv is not a valid eiruv.

The Gemara now explains the difference: If the key is found in a city, his eiruv is a valid eiruv in accordance with the opinion of Rabbi Shimon, who said: Roofs, courtyards, and enclosed fields [karpeifot] are all one domain with regard to utensils that began Shabbat in them. Accordingly, a utensil that was left on a roof at the beginning of Shabbat may be carried into a courtyard or a karpef. It is possible to transfer anything located in a city from one place to another in a similar manner. If, however, the key is found in a field, his eiruv is not a valid eiruv, in accordance with the opinion of the Rabbis, due to the prohibition to carry in a karmelit. Although carrying there is merely a shevut, the eiruv is not valid.

The discussion above constitutes one understanding of the mishna. Rabba and Rav Yosef, however, both said: Here, we are dealing with a wooden cupboard, and the tanna’im disagree with regard to the following point: The first, anonymous Sage, who rules that the eiruv is valid, holds that the cupboard is a utensil, and that there is no prohibited labor of building utensils, and similarly, there is no dismantling of utensils. Since dismantling a utensil is not included in the prohibited labor of dismantling, one may make a hole in the cupboard in order to access the food used for the eiruv. And the other Sage, Rabbi Eliezer, who invalidates the eiruv, holds that the cupboard is a tent. A wooden implement of such a large size is no longer classified as a utensil; rather, it is considered a building, and therefore it is subject to the prohibitions against building and dismantling on Shabbat.

And their dispute is parallel to the dispute between these tanna’im, as we learned in a mishna with regard to the ritual impurity of a zav: One of the unique laws of a zav is that he imparts impurity to an object simply by moving it, even if he does not touch it directly. If a zav knocked on a carriage, crate, or cupboard, even if he did not actually come into direct contact with them, they are nonetheless ritually impure because he caused them to move when he struck them. Rabbi Neḥemya and Rabbi Shimon disagree and render them pure.

What, are they not disagreeing about this point: The first Sage holds that a carriage, crate, or cupboard is categorized as a utensil, and therefore it contracts ritual impurity when a zav causes it to move; and the other Sage, Rabbi Neḥemya and Rabbi Shimon, holds that it is a tent, and a building does not contract ritual impurity in any way from a zav?

Abaye said in refutation of this proof: And do you think that this is a reasonable explanation of the mishna? Wasn’t it taught in a baraita: If a zav shook a real tent, and it moved, it is ritually impure; and if he shook a utensil and it did not move, it is ritually pure? This indicates that the critical factor is not whether the article is classified as a tent or as a utensil but whether or not it actually moves when shaken. Furthermore, it was taught in the latter clause of that baraita: And if they moved, they are ritually impure, and this is the principle: If a utensil or a tent moved due to the direct force of the zav, it is ritually impure. But if it moved due to vibrations, e.g., if the zav knocked on the floor or on the platform upon which the object is located, and the vibrations of the floor or the platform caused the object to move, it is ritually pure, as it was not moved by the direct force of the zav. Once again, the determining factor is not the object’s classification as a tent or a utensil but whether it was actually moved by the zav.

Rather, Abaye said that the dispute between the first tanna and Rabbi Neḥemya and Rabbi Shimon should be understood as follows: All agree that movement due to the direct force of the zav causes the object to become ritually impure, whether it is a tent or a utensil. Conversely, if the movement was due to vibrations of the floor or base, it is ritually pure. And here, we are dealing with a case where the object vibrated because of the direct force of the zav, i.e., where he banged upon the object itself, causing it to vibrate but not to move. And the tanna’im disagree with regard to the following point: The first Sage holds that this, too, is considered movement; and the other Sage, Rabbi Neḥemya and Rabbi Shimon, holds that vibration is not considered movement. Therefore, Abaye rejects Rabba and Rav Yosef’s proof for their explanation of the mishna.

The Gemara therefore proceeds to ask: If so, how is the mishna with regard to eiruv to be interpreted? The Gemara answers: Abaye and Rava both said: We are dealing here with a lock that is tied with a leather strap, and a knife is required to cut it if there is no key.

The anonymous first tanna holds in accordance with the opinion of Rabbi Yosei, who said: All utensils may be moved on Shabbat, except for a large saw and the blade of a plow. Consequently, one may take a knife, cut the strap, and remove his eiruv from the cupboard.

And Rabbi Eliezer holds in accordance with the opinion of Rabbi Neḥemya, who said: Even a cloak, and even a spoon, which are certainly used only for activities permitted on Shabbat, may be moved on Shabbat only for the purpose of their ordinary use. The same applies to a knife, which may be moved only in order to cut food, but not for any other purpose. Consequently, one cannot cut the strap around the lock of the cupboard, and therefore his eiruv is invalid unless the key is located in town and he can transport it via courtyards.

MISHNA: If one’s eiruv rolled beyond the Shabbat limit, and he no longer has access to his eiruv since he may not go beyond his limit, or if a pile of stones fell on it, or if it was burnt, or if the eiruv was teruma and it became ritually impure; if any of these occurrences took place while it was still day, prior to the onset of Shabbat, it is not a valid eiruv, since one did not have an eiruv at twilight, which is the time one’s Shabbat residence is established. However, if any of these occurred after dark, when it was already Shabbat, it is a valid eiruv, as it was intact and accessible at the time one’s Shabbat residence is determined.

If the matter is in doubt, i.e., if he does not know when one of the aforementioned incidents occurred, Rabbi Meir and Rabbi Yehuda say: This person is in the position of both a donkey driver, who must prod the animal from behind, and a camel driver, who must lead the animal from the front, i.e., he is a person who is pulled in two opposite directions. Due to the uncertainty concerning his Shabbat border, he must act stringently, as though his resting place were both in his town and at the location where he placed the eiruv. He must restrict his Shabbat movement to those areas that are within two thousand cubits of both locations.

Rabbi Yosei and Rabbi Shimon disagree and say: An eiruv whose validity is in doubt is nevertheless valid. Rav Yosei said: The Sage Avtolemos testified in the name of five Elders that an eiruv whose validity is in doubt is valid.

GEMARA: We learned in the mishna: If one’s eiruv rolled beyond the Shabbat limit prior to the onset of Shabbat, it is not a valid eiruv. Rava said: They only taught this in a case where one established his eiruv at the edge of his town’s Shabbat limit and the eiruv rolled more than four cubits outside that limit; however, if it remained within four cubits of the Shabbat limit, it is a valid eiruv. The principle is that one who places his eiruv in a particular location has four cubits around it, since he has established his Shabbat residence there.

The mishna continues: If a pile of stones fell on the eiruv prior to the onset of Shabbat, it is not a valid eiruv. It might enter your mind to say that the mishna is referring to a case where if one wanted he could take the eiruv, i.e., where it is physically possible to clear the stones and remove the eiruv from underneath them. The only reason he cannot do so is because of the rabbinic prohibition to handle items that are set-aside, such as stones, on Shabbat.

If so, let us say that the mishna is not in accordance with the opinion of Rabbi Yehuda HaNasi. As, if you say that it is in accordance with the opinion of Rabbi Yehuda HaNasi, there is a difficulty: Didn’t he say that with regard to anything prohibited due to rabbinic decree, they did not issue the decree to apply during twilight? The prohibition to handle items that are set-aside is also a rabbinic decree, and therefore, according to Rabbi Yehuda HaNasi, since the eiruv was accessible at twilight, it should be valid.

The Gemara rejects this argument: Even if you say that the mishna is in accordance with the opinion of Rabbi Yehuda HaNasi, we can say that this ruling was necessary only in a case where a hoe or a spade would be required in order to remove the eiruv from under the stones, i.e., one would have to dig, which is a Shabbat labor prohibited by Torah law, not only by rabbinic decree.

The Gemara comments: And both rulings, the ruling concerning an eiruv that rolled beyond the Shabbat limit and the ruling concerning an eiruv that became buried under a pile of rocks, are necessary. As, if the mishna had only taught the case of the eiruv that rolled away, we might have said that the eiruv is invalid because it is not near him, but if a pile of rocks fell on the eiruv, since it is near him, you might say that it should be a valid eiruv, as one does not actually have to eat the eiruv.

And conversely, if the mishna had only taught the case where a pile of rocks fell on the eiruv, we might have said that the eiruv is invalid because it is covered, but in the case where it rolled away, since sometimes a wind comes and brings it back, you might say that it should be a valid eiruv. Therefore, it was necessary to teach both cases.

The mishna further states: Or if the eiruv was burnt, or if the eiruv was teruma that became ritually impure before Shabbat, it is not a valid eiruv. The Gemara asks: Why do I need to teach these two cases? The essential point of both cases is the same: The eiruv is no longer fit to be eaten. The Gemara answers: The mishna taught the case where the eiruv was burnt