Eruvin 15A

איתמר: לחי העומד מאליו, אביי אמר: הוי לחי, רבא אמר: לא הוי לחי.

It was stated that the amora’im disagreed about a side post that stands by itself, i.e., a side post at the entrance to an alleyway that was not put there for the express purpose of permitting one to carry on Shabbat. Abaye said: It is a valid side post. Rava said: It is not a valid side post.

היכא דלא סמכינן עליה מאתמול, כולי עלמא לא פליגי דלא הוי לחי. כי פליגי היכא דסמכינן עליה מאתמול. אביי אמר: הוי לחי, דהא סמכינן עליה מאתמול. רבא אמר: לא הוי לחי, כיון דמעיקרא לאו אדעתיה דהכי עבידי, לא הוי לחי.

The Talmud first narrows the scope of the dispute: In a place where the inhabitants of the alleyway did not rely on it from yesterday, e.g., the alleyway had another side post that fell down on Shabbat, all agree that it is not a valid side post. Where they disagree is in a case where they relied on it from yesterday. Abaye said: It is a valid side post, as they relied on it from yesterday. Rava said: It is not a valid side post; since it was not originally erected for this purpose, it is not considered a valid side post.

קא סלקא דעתך כי היכי דפליגי בלחי פליגי נמי במחיצה.

The Talmud comments: It might enter your mind to say that just as they disagree with regard to a side post, they also disagree with regard to whether a partition that was not erected to serve that function is considered a valid partition.

תא שמע: העושה סוכתו בין האילנות ואילנות דפנות לה ⁠— כשירה. הכא במאי עסקינן, שנטען מתחילה לכך. אי הכי פשיטא! מהו דתימא: ליגזור דילמא אתי לאישתמושי באילן, קא משמע לן.

Come and hear a proof based upon what we learned in the following Mishnah: With regard to one who makes his sukka among the trees, and the trees serve as its walls, it is a valid sukka. This proves that the trees function as partitions even though they were not erected for this purpose. The Talmud responds: With what are we dealing here, in this Mishnah? To a case where he planted the trees from the outset for this purpose. The Talmud asks: If so, it is obvious that the trees constitute valid walls. The Talmud answers: Lest you say the rabbis should issue a decree to prohibit using a sukka with trees as its walls, due to a concern that perhaps one will come to use the tree on the Festival and detach a branch or leaf in the process, the Mishnah therefore teaches us that no such decree was made and the sukka is permitted.