We now learn the third Mishnah in the tenth chapter of Maseches Eruvin, which deals with a person reading from a scroll on Shabbos when the scroll rolls out of his hand into the public domain - and how he should act so that the honor of the scroll is preserved without transgressing the prohibitions of carrying out and carrying in.
"HaKorei b'sefer al ha'iskufah": - One who is reading from a scroll on the threshold.
In those days scrolls were written in the form of a megillah, wound around a rod. An iskufah is a sort of landing or porch in front of the house, and according to Abaye's explanation in the Gemara its status is that of a karmelis - it is not a private domain but a karmelis.
What is the case? The person is standing on the threshold, holding the rod around which the scroll is wound, and the scroll "nisgalgel miyado" - it slipped from his hand and unrolled, and its end reached into the public domain. The Mishnah says: "golelo etzlo" - he may roll the scroll back toward himself.
Why is this permitted? Even if he were standing in a private domain, where taking an object from the public domain into a private domain is normally a Torah prohibition, here - since the end of the scroll is still in his hand - this is not a Torah prohibition but only a rabbinic one. And in our case he is standing in a karmelis, and bringing an object in from the public domain to a karmelis is only rabbinic to begin with, and in addition the scroll is held in his hand - therefore they permitted him to roll it back to himself.
"HaKorei b'rosh hagag": - One who is reading on top of the roof.
The second case: a person is reading on the roof, which is a private domain, and the scroll rolls out of his hand downward, into the airspace of the public domain. Here the law depends on where the scroll has reached:
"Ad shelo higia l'asarah tefachim" - as long as it has not reached the lowest ten tefachim of the public domain, "golelo etzlo" - he rolls it back to himself. For above ten tefachim in the public domain the status is that of an exempt area, and it is not considered the public domain proper. From an exempt area into a private domain it is permitted to transfer even at the outset, and therefore he may roll the scroll back up to himself.
"Mishehigia l'asarah tefachim" - once the scroll has reached within ten tefachim of the ground, he is not permitted to roll it back upward. Rather, "hofcho al haksav" - he turns the scroll over so that the written side of the parchment faces the wall and is not exposed. In this way at least a measure of protection for the honor of the scroll is achieved.
The Gemara's addition: a slanted wall and a wall that is not slanted:
The Gemara explains that several words are missing from the wording of the Mishnah, and it should be read as follows - "Bameh devarim amurim?" - in which case was it said that he must turn the scroll over rather than roll it back to himself:
A slanted wall: a wall that leans, that does not stand at a ninety degree angle, and therefore the bottom of the scroll comes to rest upon it. As long as it is above ten tefachim there is no problem, since that is an exempt area, even though the scroll rests there; but if it reached within ten tefachim of the ground and it comes to rest, it turns out that the scroll is resting in the public domain, and at that point he is not permitted to roll it back up, but rather he turns it over onto its writing.
A wall that is not slanted: a wall that descends straight down at a ninety degree angle, where the scroll does not lean against any spot and does not come to rest in the public domain - "golelo etzlo" - and he may roll it back toward himself.
The opinions of the Tannaim in the Mishnah:
Rabbi Yehudah: "Even if it is removed from the ground by no more than the thickness of a needle - he rolls it back to himself." Even if the scroll descended all the way down and is raised above the ground by no more than the width of a needle, since it is not resting on the ground but suspended in the airspace of the public domain, it is permitted to roll it back. This is also the opinion according to which the wording of the Mishnah is resolved with the distinction between a slanted wall and a wall that is not slanted.
Rabbi Shimon is more lenient: "Even if it is on the ground itself - he rolls it back to himself." Even if the scroll is lying on the ground itself, it is permitted to roll it back upward.
What is the basis of Rabbi Shimon's opinion? As was explained at the outset, since the top of the scroll is held in his hand, there is no discussion here of a Torah prohibition at all, but only of a rabbinic one. And Rabbi Shimon holds that nothing whose prohibition is rabbinic stands in the way of the honor of the holy writings.
In summary: since we are dealing with a scroll, even though there is here a transfer from the public domain into a private domain, since one end is held in his hand this is not a Torah transgression but a rabbinic one - and this prohibition is set aside for the honor of the holy writings, and he is permitted to roll the scroll back to himself.