In that Rashi he explains that the Gemoro is difficult based on what we had learned earlier in the first Perek in 7b
My question is that if you remember in that Sugya there were actually 8 Tanoim who held of that Halochoh and the Gemoro there goes through the Mekoiros of all of them while here in our Sugya Rashi only cites 5 of them missing out 3 being Rebbi,Rebbi Yoshiah and Beis Shammai why did he not bring those 3 also in his commentary here in Daf 21b and why is he implying its only difficult on 5 of them and not all 8 of them.
Boruch Kahn, London England
Reb Baruch, Shalom Aleichem! How are you?
1) The answer to your question is that there are different levels of Diras Keva. The Rishonim write this in the first Perek. See Tosfos 7b, DH Kulhu, who writes that the different Shitos are not equal, and see Ba'al ha'Me'or (4a in pages of the Rif) who writes that there are all sorts of different ways of a Sukah being considered permanent.
2) We may use a similar way of thinking to understand Rashi on 21b. Here, Rashi mentions only more significant levels of permanence. It is easiest to understand why he did not mention Beis Shamai, because they require only a very slight level of permanence (that one get most of the body and the head in). This is a much lower level of Keva than is discussed by Rebbi Yehudah and Rebbi Shimon, about whom the Sugya on 21b is discussing. Rebbi Yehudah says that even if the Sukah is over 20 Amos high, it is still valid, even though this is a high level of permanence, and Rebbi Shimon also requires four walls, which is more permanent than the Sukah of Beis Shamai.
3) The Sukah of Rebbi is also a lower level form of Keva because it is only 4 Amos by 4 Amos, which, even though it is Halachically a house, it is a very small house.
The Keva required by Rebbi Yoshiya is also a weak sort of permanence, because he says that if the sun can get in through the walls, this is not Keva. One does not require very sophisticated walls to prevent the sun from coming in.
4) In contrast, the other Shitos which Rashi mentions are referring to a higher standard of fixed-ness. Raban Gamliel says that even though the Sukah is fine on a quiet day, it is not Keva if it gets blown away in the wind. Rebbi Eliezer says that even though the Sukah is strong, it is not Keva if it has no roof. Finally, Acherim say that a round Sukah is not considered Keva even though it might be a strong building.
5) To summarize, the degree of permanence that the Sugya on 21b is discussing is a better level of permanence than on 7b, and this is why Rashi mentioned only the Shitos that require this higher level of Keva.
Kol Tuv,
Dovid Bloom