פירוש על בבא בתרא 3:15

Guda: A wall.

They assumed, "What is a mechitzah? A wall": Even though [this understanding] stands in the end, it said, "they assumed," since this version is not the main one, due to the challenge from later. And that which it answers [that challenge] is a mere deflecting (not fully satisfying). But the law is like the second version, and so does Rabbenu Tam decide; and once it [used] the expression, "They assumed," here - it also used it in the other version. And further (another answer is that) RI says that in the first chapter of Nedarim (11a), "they assumed," is found even though it stands in the end and it concludes like it.

As it was taught (in the mishnah) [in a baraita]: Since the tanna (teacher) called a wall, mechitzah.

As it was taught in a baraita, "A partition of a vineyard that was breached": There are texts that have the version, "as it was taught in a mishnah." But Rabbenu Tam says that this is not a mishnah in any place. (However if that is so,) how did [the Gemara] conclude from it in [the chapter entitled] HaGozel Kamma (Bava Kamma 100a, ff.) that Rabbi Meir judges [with] the law of garmi. As if it were a mishnah, it would come out well, since an unspecified mishnah is [from] Rabbi Meir! So Rabbenu Tam says that it implied that it goes according to Rabbi Meir based on the force of [the] mishnah in Kilayim (7:4) that it brings there, "One who causes his vine to overhang his fellow’s grain, the grain is rendered prohibited and he is liable for it." And that is an unspecified mishnah. Hence this too that is taught, "it is rendered prohibited," is [from] Rabbi Meir. But we should not elaborate more here.

Mechitzat (A partition of) a vineyard: That is next to his fellow's cultivated field [of grain].

He may say to him, "Build a partition": [Meaning] to the owner of the vineyard, and he is the damager. For the four ells' distance that they said to make is because of the work on the vineyard, as we said in the second chapter later (Bava Batra 26a). And [this is the case] even according to Rabbi Yose, who said later (Bava Batra 18b, 25b), it is upon the damaged to distance himself - behold, Rav Ashi said later (Bava Batra 25b), "Rabbi Yose concedes in [the case] of his arrows (when the damager clearly causes the damage)." And RI says that this is why it teaches, "He may say to him, 'Build a partition'"; and it does not teach, "He is liable to build the partition: Since he must warn him. And if he does not warn him, he is not liable for the loss. And that is also why it twice [mentions], "[If] it was breached, he may say to him, 'Build a partition.'" [Because] even when it was breached the second time, he must [still] warn him. For he does not figure that he liable to build it all the time. But if it is breached a third time, RI was in doubt whether he must warn him all the time, or maybe he does not need to warn him any more. But Rabbenu Tam explains that this is why it [mentions it] twice: To have us understand that if there is an addition of a two hundredth between what [grew] during the first breach and what [grew] during the second breach - that they do not combine to create a prohibition; and they [start over] each time. And I elaborated [about this] in [the chapter entitled] HaGozel Kamma (Bava Kamma 100a, ff.) and in the chapter [entitled] BeMeh Eshah (Shabbat 65a.)

He may say to him: To the owner of the vineyard, "You must build a fence, so that your vines do not prohibit the produce of the field, on account of forbidden mixtures." For the owner of the field [planted] his seeds adjacent to the fence, as we learned (in the mishnah, Bava Batra 26a), "If there was a fence between them, this one may place his trees close to the fence from here, and that one may place his produce close to the fence from there." But when there is no fence there, we learned in the second chapter [as well], "A person may not plant a tree near the field of another unless he distances it four ells" - enough for the work of the vineyard. As [otherwise] it is [a case of] forbidden mixtures.

If it was breached again, he may say to him: To make a partition.