Abaye uses a legal fiction to pretend that there are walls for this sukkah. The roof of the exedra, upon which the skhakh rests, is looked at if its edges goes down to the ground and fill up the space needed for a wall.
Rava does not invoke this legal fiction.
Rava raises a rhetorical difficulty on Abaye. If there is a regular sukkah with three walls and the middle wall falls down, we could look at the edges of the existent two walls as if they fictiously go down to the ground and create a third wall. This would be absurd for it would in essence validate all two walled sukkot.
Abaye agrees that this sukkah is invalid because with two parallel walls it is like an alley-way. There is no way to consider the third wall as fictitiously existing.
Today’s section continues to deal with the “exedra” an open but covered area usually found inside a courtyard, outside of the house. In yesterday’s section we learned that Abaye holds that the edge of the roof of the exedra can fictitiously be extended below such that the sukkah found in the courtyard can be considered as if it has walls. Rava disagreed.
The Talmud attempts to correlate between Abaye and Rava’s dispute concerning the sukkah in the exedra with a similar dispute between Rav and Shmuel concerning carrying on Shabbat in an exedra found in a field. Rav and Shmuel’s dispute concerns an “exedra” in the field. Clearly this cannot be at all similar to the structure found in Roman-style courtyards. Rather, it seems that these amoraim have used this word to refer to a covered structure without walls standing in a field. The cover was probably made for shade. Rav holds that one can carry throughout the entire exedra on Shabbat because we can consider this to be a closed area. Shmuel says we do not consider the roof to be extended to the ground. Therefore, this is an open area and on Shabbat one can carry only up to four cubits.