Sukkah 2A

סוכה שהיא גבוהה למעלה מעשרים אמה ⁠— פסולה. ורבי יהודה מכשיר.

Mishnah: A sukka, i.e., its roofing, which is the main and most crucial element of the mitzva, that is more than 20 cubits high is unfit. R' Yehuda deems it fit.

ושאינה גבוהה עשרה טפחים, ושאין לה (שלשה) דפנות, ושחמתה מרובה מצלתה ⁠— פסולה.

Similarly, a sukka that is not even 10 handbreadths high, and one that does not have 3 walls, and one whose sunlight that passes through its roofing is greater than its shade are unfit.

גמ׳ תנן התם: מבוי שהוא גבוה מעשרים אמה ⁠— ימעט. רבי יהודה אומר: אינו צריך.

Talmud: We learned a similar halakha in a Mishnah there, in tractate Eiruvin (2a): In the case of an alleyway that is higher than 20 cubits, i.e., the beam that was placed across the end of an alleyway that opens into a public domain in order to permit carrying within the alleyway on Shabbat is higher than 20 cubits, one must diminish the height of the beam in order to permit carrying within the alleyway. R' Yehuda says he need not do so, and although the beam lies higher than 20 cubits, the alleyway is qualified to permit carrying within.

מאי שנא גבי סוכה דתני פסולה, ומאי שנא גבי מבוי דתני תקנתא?

Given the seeming similarity between the two cases, that of the sukka and that of the alleyway, the Talmud asks: What is different with regard to a sukka where the Mishnah teaches that it is unfit, and what is different with regard to an alleyway where the Mishnah teaches the method of rectification, that one must diminish the height of the cross beam? Why was a solution not suggested in the case of a sukka?

סוכה דאורייתא, תני פסולה. מבוי דרבנן, תני תקנתא.

The Talmud answers: With regard to sukka, since it is a mitzva by Torah law, the Mishnah teaches that it is unfit, as, if it is not constructed in the proper manner, no mitzva is fulfilled. However, with regard to an alleyway, where the entire prohibition of carrying is only by rabbinic law, the Mishnah teaches the method of rectification, as the cross beam comes only to rectify a rabbinic prohibition but does not involve a mitzva by Torah law.