Eruvin 3B

חלל סוכה תנן, חלל מבוי תנן.

as that which we learned in the Mishnah, with regard to the unfitness of a sukka higher than 20 cubits, is referring to the interior space of the sukka; and that which we learned in the Mishnah, that a cross beam spanning an alleyway that is more than 20 cubits must be lowered, is referring to the space at the entrance of the alleyway beneath the cross beam.

אמר ליה רב פפא לרבא, תניא דמסייע לך: מבוי שהוא גבוה מעשרים אמה יותר מפתחו של היכל ימעט. והיכל גופו חללו עשרים.

Rav Pappa said to Rava: A baraita was taught that supports your opinion: If the cross beam spanning the entrance of an alleyway is higher than 20 cubits off the ground, greater than the entrance of the Sanctuary, one must diminish its height. And the space of the entrance of the Sanctuary itself was 20 cubits high, and its roof was higher than 20 cubits. Apparently, the 20 cubits mentioned with regard to a sukka and an alleyway refers to the space beneath the roofing and the cross beam.

איתיביה רב שימי בר רב אשי לרב פפא: כיצד היה עושה ⁠— מניח קורה משפת עשרים ולמטה!

Rav Shimi bar Rav Ashi raised an objection to Rav Pappa from the Tosefta: How precisely would he do it? He places the cross beam from the edge of 20 cubits and below. Apparently, the entire cross beam must be within 20 cubits of the ground, and if any part of it rises above 20 it is unfit.

אימא: ״ולמעלה״. והא ״למטה״ קתני!

Rav Pappa replied: Emend the Tosefta and say: From the edge of 20 cubits and above. Rav Shimi retorted: But isn’t it taught: And below? What justification is there to completely reverse the meaning of the Tosefta?