QUESTION: Rav and Rebbi Yochanan argue about the status of the top of a wall between two Chatzeros, when the area of the top of the wall is less than four by four Tefachim. Rav says that the residents of both Chatzeros have the right to use the top of the wall (since it is not an independent domain in itself). Consequently, it is forbidden to carry (or move an object from its place in any other manner) even on top of the wall as in any shared domain not joined through an Eruv. Rebbi Yochanan says that each Chatzer may use the top of the wall since it is a Makom Petur.

The Gemara challenges Rebbi Yochanan's opinion from the Mishnah (76b), which says that the residents of each Chatzer may use the top of the wall, but they may not carry things between the wall and their Chatzer.

What is the Gemara's question? The Mishnah is discussing a case in which the area atop the wall is larger than four by four Tefachim, as the Mishnah explicitly states. Rebbi Yochanan, in contrast, is discussing a case in which the area atop the wall is smaller than four by four Tefachim! If, for some reason, the Gemara considers the question from the Mishnah to be a valid question on the opinion of Rebbi Yochanan, then it should also be a valid question on the opinion of Rav, because Rav says that the top of the wall may not be used at all!

(a) TOSFOS (DH Olin) answers that the Gemara does not challenge Rav's opinion from the Mishnah because it knows that Rav's reasoning is that "both domains (the two Chatzeros) have control over the top of the wall," and therefore, as a shared domain, it is forbidden to carry there. However, this reasoning applies only when the area is smaller than four by four Tefachim. When the area is four by four Tefachim or larger, it is considered an independent domain unto itself and one may carry atop it, as the Mishnah states. The Gemara's question applies only to the opinion of Rebbi Yochanan. The Gemara initially assumed that the reason Rebbi Yochanan permits the residents of both Chatzeros to carry atop the wall is because it is not convenient for either Chatzer to use the top of the wall, and therefore neither Chatzer has control over it. If that is Rebbi Yochanan's reasoning, then it should apply just the same when the top of the wall is four by four Tefachim or larger, because it is still difficult to climb up and use it, and thus it should still be considered a separate domain from each Chatzer.

The Gemara answers that Rebbi Yochanan's reasoning is not that the top of the wall is a domain unto itself because it is difficult to use that area, but because that area is a Makom Petur. When that area is wider than four Tefachim, it is no longer a Makom Petur and it is forbidden to transfer from there to the Chatzer because it is a separate domain unto itself.

(b) The CHIDUSHEI HA'RAN answers as follows. The Mishnah says that when the area atop the wall is four by four Tefachim, each person may go up to the top of the wall and eat items that were already there, as long as they do not take anything down from there. The Mishnah's wording seems to emphasize what one is permitted to do (Heter). The Gemara infers from the Mishnah's emphasis on the Heter that had the area atop the wall not been four by four Tefachim, it would have been completely forbidden to use the top of the wall, but now that it is four by four Tefachim, there is a Heter to use it.

According to Rebbi Yochanan, who says that when the area atop the wall is smaller than four by four Tefachim, one is permitted to eat on top of the wall and to bring food there, the Mishnah should have focused on the Isur and said, "One may not bring food up and eat it, but one may go up and eat food that was already there," with the emphasis on what one may not do.

QUESTION: The Mishnah (76b) states that a wall at least ten Tefachim high between two Chatzeros is considered a separate domain from each Chatzer, and the residents of each Chatzer may not carry to or from the top of the wall. The Gemara says that one may decrease the height of the wall below ten Tefachim (and thereby make it permissible for the two Chatzeros to make an Eruv together and to carry from each Chatzer to the top of the wall; see RITVA) by placing a mound of earth that is at least four Tefachim long (RASHI; according to TOSFOS (77b, DH Im), the mound of earth must also be four Tefachim wide) at the bottom of the wall. If the height from the top of the mound to the top of the wall is less than ten Tefachim, then the mound serves as an entranceway to the top of the wall and the entire length of the wall may be used. If the mound is less than four Tefachim long, then only the actual section of the wall against which the mound was placed may be used.

The Gemara asks that if a mound less than four Tefachim suffices, then it should permit the use of the entire length of the wall. If it does not suffice, then it should not permit even the small section of wall under which it lies. Ravina answers that the Gemara is referring to a case in which one removed a brick from the top of the wall. In such a case, if the removal of the brick diminished the height of the wall to below ten Tefachim only along a section of the wall that was less than four Tefachim long, then one is permitted to use that section of the wall, but not the rest of the wall.

How does this answer the Gemara's original question? What difference does it make if the decrease in height of the wall occurs as a result of raising the floor at the bottom of the wall, or lowering the height at the top of the wall? If a mound less than four Tefachim at the bottom of the wall does not help at all, then why should lowering the top of the wall by that amount help?

(a) RASHI (DH Amar Ravina) seems to explain that when the decrease occurs at the top of the wall, even though it might not be considered an entranceway (since it is less than four Tefachim), it still serves to lower the wall at that point, and thus that section of the wall may be used. However, when a mound is placed at the bottom of the wall to decrease the wall's height, the wall itself is not actually affected because it is still ten Tefachim high (when viewed from the other side). Therefore, the section directly above the mound may not be used because the wall is still ten Tefachim high.

(b) The RASHBA and ROSH (end of 7:1) explain that a mound less than four Tefachim long at the bottom of the wall is not a large enough area on which to balance oneself in order to use the top of the wall. Therefore, the wall is still considered to be ten Tefachim high. A small gap at the top of the wall, though, can easily be used by someone standing on the ground.

Perhaps the Rashba and Rosh do not accept Rashi's explanation because they understand that the Gemara is discussing a case in which one decreases the size of the wall from both sides in order to make a Halachic passageway between the two Chatzeros and permit the two Chatzeros to make one Eruv together (as the Ritva writes).

Rashi, on the other hand, understands that the Gemara is not referring specifically to two Chatzeros whose residents want to decrease the height of the wall between them, but it is referring even to the residents of a single Chatzer who want to decrease the height of a wall next to their Chatzer (regardless of what is on the other side of the wall) in order to use the top of the wall on Shabbos. Therefore, the mound placed on one side does not serve to decrease the height of the wall, since it is still ten Tefachim high on the other side.