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

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

GEMARA: We learned a similar halakha in a mishna there, in tractate Eiruvin (2a): In the case of an alleyway that is higher than twenty 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 twenty cubits, one must diminish the height of the beam in order to permit carrying within the alleyway. Rabbi Yehuda says he need not do so, and although the beam lies higher than twenty cubits, the alleyway is qualified to permit carrying within.

Given the seeming similarity between the two cases, that of the sukka and that of the alleyway, the Gemara asks: What is different with regard to a sukka where the mishna teaches that it is unfit, and what is different with regard to an alleyway where the mishna 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?

The Gemara answers: With regard to sukka, since it is a mitzva by Torah law, the mishna 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 mishna 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.

The Gemara suggests an alternative explanation: And if you wish, say instead that even with regard to matters prohibited by Torah law, it would have been appropriate for the mishna to teach a method of rectification. However, with regard to sukka, whose matters are numerous, it categorically teaches that it is unfit. Merely diminishing the height of a sukka is insufficient to render it fit; the sukka must also satisfy requirements governing its size, its walls, and its roofing. Teaching the remedy for each disqualification would have required lengthy elaboration. With regard to an alleyway, however, whose matters are not numerous, the mishna teaches the method of rectification. Once the height is diminished, it is permitted to carry in the alleyway.

§ After clarifying its formulation, the Gemara addresses the halakha in the mishna and asks: From where are these matters, i.e., the halakha that a sukka may not exceed a height of twenty cubits, derived?

Rabba said that it is derived as the verse states: “So that your future generations will know that I caused the children of Israel to reside in sukkot when I took them out of the land of Egypt” (Leviticus 23:43). In a sukka up to twenty cubits high, even without a concerted effort, a person is aware that he is residing in a sukka. His eye catches sight of the roofing, evoking the sukka and its associated mitzvot. However, in a sukka that is more than twenty cubits high, a person is not aware that he is residing in a sukka because his eye does not involuntarily catch sight of the roof, as at that height, without a concerted effort one would not notice the roofing.

Rabbi Zeira said that it is derived from here: The verse states: “And there shall be a sukka for shade in the daytime from the heat, and for refuge and cover from storm and from rain” (Isaiah 4:6). In a sukka up to twenty cubits high, a person is sitting in the shade of the sukka, i.e., the shade of the roofing; in a sukka that is more than twenty cubits high, a person is not sitting in the shade of the roofing of the sukka but rather in the shade of the walls of the sukka, as their considerable height provides constant shade, rendering the shade of the roofing irrelevant.

Abaye said to him: But if it is so that one is required to sit in the shade of the roofing of the sukka, then in the case of one who makes his sukka in Ashterot Karnayim, which is located between two mountains that prevent sunlight from reaching there, so too, it is not a fit sukka, since he is not sitting in the shade of the roofing.

Rabbi Zeira said to him: The two cases are not comparable; there, if one theoretically removes the Ashterot Karnayim mountains that obstruct the sunlight, there is still the shade of the roofing of the sukka. In that case, the sukka is properly constructed and there are only external factors that affect the sunlight. However, here, in the case of a sukka that is more than twenty cubits high, if one theoretically removes the walls of the sukka, there is no shade provided by the roofing of the sukka, since throughout the day sunlight will enter the sukka beneath the roofing from where the walls used to be.

Rava said that the halakha is derived from here: “In sukkot shall you reside seven days” (Leviticus 23:42). The Torah said: For the entire seven days, emerge from the permanent residence in which you reside year round and reside in a temporary residence, the sukka. In constructing a sukka up to twenty cubits high, a person can render his residence a temporary residence, as up to that height one can construct a structure that is not sturdy; however, in constructing a sukka above twenty cubits high, one cannot render his residence a temporary residence; rather, he must construct a sturdy permanent residence, which is unfit for use as a sukka.

Abaye said to him: But if that is so, then if he constructed a sukka with steel partitions and placed roofing over them, so too, there, say that it would not be a fit sukka, as any sukka constructed as a permanent residence would be unfit. However, there is no opinion that deems a sukka of that sort unfit.

Rava said to him in response that this is what I am saying to you: In a case where one constructs a sukka up to twenty cubits high, a height that a person typically constructs a temporary residence, when he constructs a structure of that height that is sturdy like a permanent residence, he also fulfills his obligation. However, in a case where one constructs a sukka more than twenty cubits high, a height that a person typically constructs a permanent residence, even when he constructs it in a less sturdy fashion like a temporary residence, he does not fulfill his obligation.