Sukkah 18B

אמרינן פי תקרה יורד וסותם. רבא אמר פסולה ⁠— לא אמרינן פי תקרה יורד וסותם.

as we say that the edge of the roof descends and seals the opening. The edge of the roof itself is considered as though it were a small partition that extends downward and forms a wall. Rava said: This sukka is unfit, as we do not say that the edge of the roof descends and seals.

אמר ליה רבא: אביי, לדידך דאמרת פי תקרה יורד וסותם, אפילו הפחית דופן אמצעי! אמר ליה: מודינא לך בההיא, דהוה ליה כמבוי המפולש.

Rava said to Abaye: According to you, who said: The edge of the roof descends and seals the opening like a wall, then in a case where the roofing of the sukka consists of straight beams, even if one removed the middle wall, leaving the sukka with only two parallel walls, the sukka would nevertheless be fit. Since the edge of the roof descends and seals, the legal status of that sukka is the same as one that has walls on all sides. Abaye said to him: I concede to you that in that particular case the principle: The edge of the roof descends and seals, does not apply, as it is considered like an open alleyway, through which the multitudes pass on two opposite sides. In other cases, the principle applies.

לימא אביי ורבא בפלוגתא דרב ושמואל קמיפלגי. דאתמר: אכסדרה בבקעה, רב אמר: מותר לטלטל בכולו, דאמרינן פי תקרה יורד וסותם. ושמואל אמר: אין מטלטלין בה אלא בארבע אמות, דלא אמרינן פי תקרה יורד וסותם.

The Talmud suggests: Let us say that Abaye and Rava disagree with regard to the same issue that was the subject in the dispute of Rav and Shmuel; they are merely elaborating on a fundamental dispute between other amora’im. As it was stated: Amora’im disagree with regard to a portico, which has a roof and no walls or incomplete walls, located in a field, which is a karmelit. Rav said: It is permitted to move an object throughout the entire portico, as we say that the edge of the roof descends and seals the opening, rendering the portico a private domain, as it is effectively surrounded by partitions. And Shmuel said: One may move an object in the portico only within 4 cubits, as we do not say that the edge of the roof descends and seals the opening. Therefore, the portico’s legal status is that of the surrounding field. Ostensibly, the basis of the dispute between Abaye and Rava is identical to the basis of the dispute between Rav and Shmuel.