The Gemara answers: It is different there, as there are no partitions. That alley is breached, and it is therefore fitting to compel its residents to establish some sort of partition, if only for the sake of protection. However, if there are partitions, one is not obligated to join in a merging of alleyways.

Another version of this explanation: From the side it is different, i.e., the preparation of an eiruv does not involve an adjustment to the alleyway itself, but rather it is a side issue. Consequently, an individual cannot be forced to participate in its preparation (Meir Netiv).

It is stated that the amora’im disputed this issue. Rav Ḥiyya bar Ashi said: One may build a side post from the wood of an asheira. Although it is prohibited to benefit from the tree and it must be burned, setting up a side post is a mitzva, and mitzvot were not given for benefit, i.e., the fulfillment of a mitzva is not in itself considered a benefit. And Rabbi Shimon ben Lakish said: One may build a cross beam from the wood of an asheira.

The Gemara clarifies their opinions: With regard to the one who said that one may build a cross beam from the wood of an asheira, Rabbi Shimon ben Lakish, all the more so he would permit a side post to be prepared from an asheira. And the one who said that one may build a side post from an asheira, Rav Ḥiyya bar Ashi, spoke only of a side post; but as for a cross beam, no, he did not permit one to build it from an asheira. The reason is that an asheira must be burned, and it is therefore as though its size has already been crushed. Consequently, a cross beam, which must be at least a handbreadth in size, may not be prepared from an asheira. With regard to a side post, by contrast, a minimum width suffices, which means that even wood from an asheira is fit, despite the fact that it is viewed as burned.

MISHNA: If the food in the barrel for the merging of the alleyway diminished and was less than the requisite measure, one may add a little of his own and confer possession to the others, and he need not inform them of his addition. However, if new residents were added to the residents of the alleyway, he may add food on behalf of those residents and confer possession to them, and he must inform the new residents of their inclusion in the merging of alleyways.

What is the measure of food required for a merging of the alleyways? When the residents of the alley are numerous, food for two meals is sufficient for all of them; when they are few, less than a certain number, a dried fig-bulk for each and every one of them is enough.

Rabbi Yosei said: In what case is this statement said? It is said with regard to the beginning of an eiruv, when it is initially established. However, with regard to the remnants of an eiruv, e.g., if the eiruv decreased in size on Shabbat, it remains valid if even any amount remains.

And in general they said that it is necessary to join the courtyards, even though a merging of the alleyways was already in place, only so that the halakhic category of eiruv will not be forgotten by the children, i.e., so that the next generation should be aware that an eiruv can be established for a courtyard, for otherwise they would be entirely unaware of this halakhic category.

GEMARA: The mishna stated: If the food in the barrel for the merging of the alleyway diminished and was less than the requisite measure, one may add to it without informing the others. The Gemara poses a question: With what case are we dealing here? If you say that he added the same type of food that had initially been used for the merging of the alleyway, why specify this particular case where the food decreased in measure? Even if the food was entirely finished, he should likewise not be obligated to inform them.

Rather, perhaps it is referring to two types of food, i.e., one added a different kind of food that had not been used beforehand. If so, even if it only decreased in measure, no, he should also not be permitted to add to the merging of the alleyway without informing them. As it was taught in a baraita: If the food of a merging of the alleyway was entirely finished, and he added the same type of food to it, he need not inform the other residents; however, if the amount he added was from two types of food, i.e., if the added food was different from the original, he must inform them. The Gemara assumes that the same halakha is in effect even if the food of the merging of the alleyway only decreased in measure.

The Gemara answers: If you wish, say that the mishna is referring to a case where his addition was from the same kind of food, and if you wish, say instead that it was from two kinds. The Gemara clarifies the previous statement: If you wish, say that it is referring to a case where his addition was from the same kind of food, and what is the meaning of decreased? It means that the food entirely crumbled away, so that nothing at all remained.

And if you wish, say instead that the mishna is referring to a case where his addition was from two kinds of food, for if the eiruv was entirely finished, the halakha is different. That is to say, the halakha of the baraita that he must inform the others is in effect only if the food was entirely finished, but if any of it remains there is no need to inform them of his addition, and he may confer possession of even a different kind of food.

We learned in the mishna: If other residents were added to the residents of the alleyway, he may add food for those residents and confer possession to them, but he must inform the new residents of their inclusion in the merging of the alleyway. Rav Sheizvi said that Rav Ḥisda said: That is to say, i.e., we can infer from the mishna, that Rabbi Yehuda’s colleagues disagree with him.

As we learned in a mishna that Rabbi Yehuda said: In what case is this statement, that an eiruv may be established for a person only with his knowledge, said? It is said with regard to a joining of Shabbat boundaries; however, with regard to a joining of courtyards, one may establish an eiruv for another person either with his knowledge or without his knowledge. Rabbi Yehuda’s opinion conflicts with that of the tanna of the mishna, who does not permit a person to be included in a joining of courtyards without his knowledge. The Gemara is surprised at this statement: It is obvious that they disagree; why did Rav Ḥisda find it necessary to teach us something so evident?

The Gemara answers: It was nonetheless necessary, lest you say that this ruling, i.e., that he must inform them, applies only to a courtyard that is situated between two alleyways, in which case the residents of the courtyard may join in a merging of alleyways with whichever alleyway they prefer. Consequently, he must inform them with which alleyway he prepared the merging, in case they preferred to join the other alleyway. However, with regard to a courtyard that opens into only one alleyway, the merging of the alleyway only benefits them and does not harm them in any way, and you might therefore say no, that even the tanna of the mishna concedes that it is not necessary to inform them. Rav Ḥisda therefore teaches us that the tanna of the mishna maintains that he must inform them in all cases, contrary to the opinion of Rabbi Yehuda.

The mishna continues: What is the measure of food required for a merging of the alleyways? It then specifies different amounts when the residents of the alley are numerous and when they are few. The Gemara asks: How many are numerous, and how many are considered few? Rav Yehuda said that Shmuel said: Eighteen people are considered numerous. The Gemara registers surprise: Eighteen and no more? The Gemara answers: Say: From eighteen upward.

The Gemara asks: And what is the significance of the number eighteen that he cited? Why this figure in particular? The Gemara answers that Rav Yitzḥak, son of Rav Yehuda, said: It was explained to me personally by my father, Rav Yehuda: Any case where, if one were to divide the food of two meals between them and it does not amount to the measure of a dried fig for each and every one of them, these are the very ones the tanna called numerous, and in this case food for two meals suffices for all of them. And if not, these are the very ones the tanna termed few, which means food in the measure of a dried fig is required for each of them.

And Rav Yehuda incidentally teaches us that food for two meals consists of a measure equal to eighteen dried figs. Consequently, if there were numerous residents in the alley, then eighteen dried figs, which is food sufficient for two meals, is enough for them.

MISHNA: One may join courtyards and merge alleyways with all types of food, except for water and salt, as they are not considered foods. This is the statement of Rabbi Eliezer. Rabbi Yehoshua says that a different limitation applies: A whole loaf may be used for an eiruv. With regard to a baked product even the size of a se’a, if it consists of pieces, one may not join courtyards with it. However, with regard to a loaf, even one the size of an issar, if it is whole, one may join courtyards with it.