משנה: הַסּוֹמֵךְ סוּכָּתוֹ לְכַרְעֵי הַמִּיטָּה כְּשֵׁירָה. רִבִּי יְהוּדָה אוֹמֵר אִם אֵינָהּ יְְכוֹלָה לַעֲמוֹד בִּפְנֵי עַצְמָהּ פְּסוּלָה. MISHNAH: If one supports his sukkah by the frame of the couch it is qualified. Rebbi Jehudah says, if it cannot stand by itself it is disqualified.
הלכה: אָמַר רִבִּי אִימִּי. מִשֵּׁם שֶׁאֵין מִמְּעֵי הַמִּיטָּה לִסְכָךְ עֲשָׂרָה טְפָחִים. אָמַר רִבִּי בָּא. מִשֵּׁם שֶׁאֵין מַעֲמִידִין עַל גַּבֵּי דָבָר טָמֵא. וְהָא תַנֵּי. מַעֲשֶׂה בְאַנְשֵׁי יְרוּשָׁלִַם שֶׁהָיוּ מְשַׁלְּשִׁין מִטּוֹתֵיהֶן לִפְנֵי חָלֹונוֹתֵיהֶן וְהָיוּ מְסַכְּכִין עַל גַּבֵּיהֶן. אִין תֵּימַר. מִשֵּׁם שֶׁאֵין מַעֲמִידִין עַל גַּבֵּי דָבָר טָמֵא. הֲרֵי מַעֲמִידִין עַל גַּבֵּי דָבָר טָמֵא. הֲוֵי. (לָא) [לֵית] טָעֲמָא (דִילָא) [אֶלָּא] מִשֵּׁם שֶׁאֵין מִמְּעֵי חַמִּיטָּה לִסְכָךְ עֲשָׂרָה טְפָחִים. HALAKHAH: Rebbi Immi said, because14The reason that R. Jehudah declares a sukkah disqualified if made from a couch with 4 poles and a thatched roof as canopy is that usually the canopy is not 10 hand-breadths higher than the couch on which one rests. Therefore, if this distance is clearly larger than 10 hand-breadths, also R. Jehudah accepts it as qualified. from the middle of the couch it is less than ten hand-breadths. Rebbi Abba said, because one does not base it on what is impure15“Is impure” as always means “is susceptible to impurity”; in the case of a couch the severe impurity induced by people whose body is the source of their impurity.. But was it not stated, it happened that the people of Jerusalem were lowering their couches through their windows and putting thatching over them16Tosephta 2:3, Babli 21b. The Tosephta states explicitly that the roofing was fastened to the windows which were 10 hand-breadths high.
In the last sentence the difference between (scribe) and [corrector] is between Galilean and Babylonian styles.? If you are saying, because one does not base it on what is impure, here it is put on what is impure. Therefore the reason only is because from the middle of the couch it is less than ten hand-breadths.