According to the theory of Rabbah, the 4 amos shiur is from a halacha l'moshe misinai and Rabban's argument of 4 tefachim (Makom Chashuv/Chatzitza) is based on Sevara. However, on daf hey the gomora said that Shiurim, Chatzitzos and Mechitzos are halacha l'moshe misinai. Why do we therefore not say here that the 4 tefach Chatzitza is based on halacha l'moshe misinai?
Benjamin Horne
London, England
We do not say that something is a Halachah l'Moshe mi'Sinai unless we have a good reason. There is nothing to force us to say that 4 Tefachim is a Halachah l'Moshe mi'Sinai, because there is a good logical reason for why a mere 4 Tefachim of invalid Sechach in the center should render the Sukah Pasul -- namely, because of "Haflagah" (it splits the Sukah into two parts). This is the reason that Rabah gave to the Rabanan on 17b as being their rationale for invalidating 4 Tefachim of Sechach Pasul, and the Rabanan never denied this. As Rashi writes (17b, DH Ela), 4 Tefachim is always considered an important place (for instance, if an area is 4 X 4 Tefachim, this is a sufficient size for it to be considered a private domain on Shabbos; see Shabbos 6a, -DB), so there is no reason to require a Halachah l'Moshe mi'Sinai to say that 4 Tefachim of invalid Sechach splits the Sukah into two Sukahs. In contrast, the source that 4 Amos should be the amount for Sechach Pasul can only be a Halachah l'Moshe mi'Sinai.
Kol Tuv,
Dovid Bloom