1) "LECHI HA'OMED ME'ELAV"
QUESTION: Abaye and Rava argue whether a Lechi that was "standing by itself" (that was not built for the purpose of being a Lechi) serves as a valid Lechi or not. The Gemara teaches that Abaye and Rava follow their respective reasoning as expressed elsewhere. Abaye, who says that the Lechi is valid, maintains that a Lechi permits one to carry in a Mavoy because it serves as a Mechitzah (partition) between the Mavoy and the Reshus ha'Rabim. A Mechitzah does not have to be erected for the explicit purpose of permitting one to carry in a Mavoy. Rava argues and maintains that a Lechi permits one to carry in a Mavoy because it serves as a Heker, a distinct reminder that indicates the end of the Mavoy. Since it is a Heker, it must be made specifically for the purpose of permitting one to carry in a Mavoy.
We find Rava's opinion (that a Lechi serves as a Heker) recorded earlier in the Gemara (12b). However, where do we find that Abaye maintains that a Lechi works by serving as a Mechitzah?
(a) RASHI here gives no source for Abaye's opinion that a Lechi serves as a Mechitzah. It seems that the text of Rashi's Gemara was the same as the text of Rabeinu Chananel's, in which the word "l'Ta'amei" ("according to his reasoning") does not appear. Their text of the Gemara read, "Abaye says that Lechi works because of Mechitzah, and Rava says that Lechi works because of Heker," and the Gemara does not say that they are following their opinions as expressed elsewhere.
(b) TOSFOS (DH Abaye) explains that in the Gemara earlier (5a), one of the explanations for the argument between Abaye and Rav Yosef (regarding the width of the span of earth that must be raised beneath a Korah that is more than 20 Amos high) is that perhaps Abaye maintains that a Korah works because it is a Mechitzah. If a Korah works because it is a Mechitzah, then certainly a Lechi also works because it is a Mechitzah (see 12b).
(c) In another place (9a), Abaye and Rava argue whether it is permitted to carry "Bein ha'Lechayayim," in the area exactly adjacent to the Lechi. The BA'AL HA'ME'OR explains that Abaye prohibits one from carrying there because he maintains that a Lechi works by serving as a Mechitzah, and the inner side of the Lechi (towards the inside of the Mavoy) is the Mechitzah. Rava permits one to carry there because he maintains that a Lechi serves as a Heker. Since the Heker is made for people to distinguish the end of the Mavoy from the beginning of the Reshus ha'Rabim, the outer side of the Lechi is the Heker. Accordingly, Abaye and Rava's argument in the Gemara here is based on the same question: Does a Lechi work because it is a Mechitzah or because it is a Heker?
(The Ba'al ha'Me'or concludes that since the Halachah follows Abaye in the case of Lechi ha'Omed me'Elav, the Halachah should also follow Abaye in the case of carrying Bein ha'Lechayayim, because the two rulings are based on the same underlying reason. However, the other Rishonim do not accept the Ba'al ha'Me'or's explanation of the Gemara here, and therefore they rule like Rava in the case of Bein ha'Lechayayim.)