Sukkah 28A

התם הוא דמבטל, אבל הכא דלא מבטל ⁠— לא.

The Talmud answers: There is a difference between the case of the shutter and the case of the sheet. There, in the case of the shutter, where he negates it by shuttering the window, it is considered part of the building and it is therefore prohibited. However, here, in the case of the sheet, where he does not negate it, as he plans on removing it, no, it is not necessarily prohibited.

תנו רבנן: מעשה ברבי אליעזר ששבת בגליל העליון, ושאלוהו שלשים הלכות בהלכות סוכה, שתים עשרה אמר להם: שמעתי, שמונה עשרה אמר להם: לא שמעתי. רבי יוסי ברבי יהודה אומר, חילוף הדברים: שמונה עשרה אמר להם: שמעתי, שתים עשרה אמר להם: לא שמעתי.

The Talmud relates a similar incident. A baraita states: There was an incident involving R' Eliezer, who stayed in the Upper Galilee, and the people there asked him 30 halakhot in the halakhot of sukka. In response to 12, he said to them: I heard an answer from my teachers, and he related what he heard. In response to the other 18, he said to them: I did not hear an answer. R' Yosei, son of R' Yehuda, says: It was the reverse of these matters. In response to 18 he said to them: I heard an answer; in response to the other 12 he said to them: I did not hear an answer.

אמרו לו: כל דבריך אינן אלא מפי השמועה! אמר להם: הזקקתוני לומר דבר שלא שמעתי מפי רבותי. מימי לא קדמני אדם בבית המדרש, ולא ישנתי בבית המדרש לא שינת קבע ולא שינת עראי, ולא הנחתי אדם בבית המדרש ויצאתי, ולא שחתי שיחת חולין, ולא אמרתי דבר שלא שמעתי מפי רבי מעולם.

They said to him: Are all the matters that you know only from what you heard? Don’t you say any matters on your own? He said to them: Now you forced me to say a matter that I did not hear from my teachers, as I must describe my character traits and the manner in which I conduct myself. In all my days, no person ever preceded me into the study hall, as I am always first to arrive; and I never slept in the study hall, neither substantial sleep nor a brief nap; and I never left anyone in the study hall and exited, as I was always last to leave; and I never engaged in idle conversation; rather, I discussed only necessary matters or matters of Torah; and I never said anything that I did not hear from my teacher. That is why he did not answer those questions that his teacher did not address.