Talmud - Eruvin 50b עירובין נ׳ ב׳

And the other Sage, Rabbi Yoḥanan, maintains: He intends to bring a large offering of eighty loaves, and therefore none of the loaves are consecrated.

Abaye said: They only taught Rav’s ruling that one cannot establish residence beneath a tree without precisely defining a particular location, with regard to a tree beneath which there are at least twelve cubits. However, with regard to a tree beneath which there are not twelve cubits, he can establish residence there, as at least part of his residence is conspicuous. In that case, there is a partial overlap between the middle four cubits beneath the tree and the four cubits nearest him and the four cubits farthest from him, and consequently each necessarily contains at least part of his residence.

Rav Huna, son of Rav Yehoshua, strongly objects to this: From where is it ascertained that he designates his residence in the four middle cubits, so that there is a partial overlap with both the nearest and the farthest cubits; perhaps he designates it in the four cubits on this side or in the four cubits on the other side? Since he does not know which location he designated as his residence, he did not establish residence anywhere beneath the tree.

Rather, Rav Huna, son of Rav Yehoshua, said: Abaye’s statement must be emended. They taught this only with regard to a tree that has at least eight cubits beneath it. However, with regard to a tree that has only seven cubits beneath it, even if one did not establish a particular location, he acquires residence, as at least part of his residence is conspicuous, as any four cubits must include at least one cubit of his residence.

With regard to the dispute between Rav and Shmuel, the Gemara notes that one baraita was taught in accordance with the opinion of Rav and another baraita was taught in accordance with the opinion of Shmuel.

The Gemara elaborates. A baraita was taught in accordance with the opinion of Rav: With regard to one who was coming along the way on Shabbat eve, and it grew dark while he was traveling, and he was familiar with a tree or a fence within two thousand cubits of his current location, and he said: My residence is beneath that tree, he has not said anything of legal consequence. However, if he said: My residence is in such-and-such place, he walks until he reaches that place. Once he reached that place that he established as his residence, he walks through all of it and another two thousand cubits beyond it.

In what case are these matters, that he establishes four cubits as his residence, and another two thousand cubits in each direction, stated? In a case where he selected a well-defined, clearly demarcated place, i.e., a case where he established residence on a mound ten handbreadths high, and its area ranges from a minimum of four cubits to a maximum of two beit se’a.

And, likewise, that is the halakha when he establishes residence on a plain ten handbreadths deeper than the surrounding area, and its area ranges from a minimum of four cubits to a maximum of two beit se’a. However, if he selected a place that is not defined, e.g., in the middle of a plain, he does not establish residence, and accordingly he has only four cubits in which to move.

If two people were walking together, one of whom is familiar with a particular location in the distance, and one of whom is not familiar with it, the one who is not familiar with it entrusts his right to designate his residence to the one who is familiar with it, and the one who is familiar with it says: My residence is in such-and-such place.

In what case are these matters, that he acquires four cubits as his residence and another two thousand cubits in each direction, stated? In a case where he defined the four cubits that he seeks to establish as his residence. However, if he did not define the four cubits that he seeks to establish as his residence, he may not move from his current place, as neither did he seek to establish residence there, nor did he acquire it in the location he sought to establish residence. This baraita is in accordance with the opinion of Rav that one who fails to designate the four cubits he seeks to establish as residence has no residence at all.

Gemara poses a question: Let us say that this baraita is a conclusive refutation of the opinion of Shmuel? The Gemara answers: There is no difficulty, as Shmuel could have said to you: With what are we dealing here? We are dealing with a special case, where from the place he is standing to the trunk of the tree there is a distance of two thousand and four cubits, so that if you were to establish residence on the other side of the tree, it would be situated outside his Shabbat limit.

Consequently, if he designated his four cubits on the near side of the tree he may go there; and if not, he may not go from the place he is standing. In other words, since he did not establish residence in a particular location, the concern is that he sought to establish it beyond his two thousand cubit limit.

A baraita was taught in accordance with the opinion of Shmuel. If one erred and established an eiruv in two directions at once, for example, if in his ignorance he imagined that it is permitted to establish an eiruv in two directions, that he may extend the distance that he may walk on Shabbat in two opposite directions, or if he said to his servants: Go out and establish an eiruv for me, without specifying the direction, and one established an eiruv for him to the north, and one established an eiruv for him to the south, he may walk to the north as far as he is permitted go based on his eiruv to the south, and he may walk to the south as far as he is permitted go based on his eiruv to the north. In other words, the assumption is that he established residence in both directions based on the eiruv in each direction, and he must therefore take both into consideration before moving.

And consequently, if each eiruv was placed two thousand cubits in opposite directions placing him in the middle of the limit, he may not move from his current location, as it is prohibited to venture beyond either limit. Apparently, even if one did not establish residence in a particular location, as in this case he has acquired residence in both places, nonetheless, the halakha is that residence has been established in his current location, in accordance with the opinion of Shmuel.

The Gemara poses a question: Let us say that this baraita is a conclusive refutation of the opinion of Rav? The Gemara answers: This baraita indeed differs with Rav’s ruling. Nevertheless, his opinion is not disqualified, as Rav himself had tanna status and therefore, unlike later amora’im, could disagree with opinions of tanna’im.

We learned in the mishna that if, however, he said: My residence is at the trunk of the tree, he established residence there, and he may walk from the place that he is standing to the trunk of the tree, up to two thousand cubits, and from the trunk of the tree to his house another two thousand cubits. Ultimately, he may walk after nightfall a total distance of four thousand cubits.

Traduction : The William Davidson digital edition of the Koren Noé Talmud, with commentary by Rabbi Adin Even-Israel Steinsaltz.

Eruvin 50b examines eruv boundaries under a tree, debating how many cubits are needed to establish a valid Shabbat residence there.