A DIVIDING WALL
(Mishnah): Partners who want to make a Mechitzah (divider) in the yard build the wall in the middle.
They build like the local custom, with uneven stones, even stones, half-bricks or bricks.
When they use uneven stones, each gives three Tefachim of his property for the wall;
When they use even stones, half-bricks or bricks, each gives two and a half Tefachim; two Tefachim, or one and a half Tefachim, respectively.
Therefore, if the wall falls, the area it occupied and the stones belong to both neighbors.
Similarly, in a garden, in a place where people make a wall, one neighbor can force the other;
In a valley, a place where people don't make a wall, one neighbor cannot force the other.
If he wants to build the wall himself, he builds on his side, and makes a Chazis (sign). If the wall falls, the area and stones are his.
If they both build the wall, they build in the middle, and make a Chazis on each side;
If the wall falls, the area and stones belong to both of them.
IS HEZEK RE'IYAH CONSIDERED DAMAGE?
Version #1 (Gemara): We are thinking that 'Mechitzah' means wall, as in the following Beraisa.
(Beraisa): If the Mechitzah of a vineyard (separating between it and a grain field) breaks, we tell the owner to repair it. If it breaks again, we tell him to repair it;
If he decided not to repair it, the grapes and grain become forbidden; and he must pay for the grain.
(Inference): The Mishnah says that they wanted to build a wall. If not, one could not force the other!
This teaches that Hezek Re'iyah (invasion of privacy) is not considered damage.
Question: Perhaps 'Mechitzah' means division, as in "Mechetzas (the portion of) ha'Edah." Once the partners agree to divide, they must build a wall!
This would teach that Hezek Re'iyah is considered damage.
Answer: If so, the Mishnah should have said 'who want to divide', and not 'who want to make a division'.
Counter-question: If Mechitzah means wall, the Mishnah should have said 'they build it', and not 'they build the wall'!
Answer: Had it said 'they build it', one might have thought that 'it' is a Mesifas (Rashi - a divider composed of stakes in the ground; Tosfos - a wall with windows). 'The wall' teaches that they must build a proper wall.
(Mishnah): They build the wall in the middle...
Question: This is obvious!
Answer: It must teach about when one neighbor persuaded the other neighbor to build it.
One might have thought that he agreed only to (a thin wall, which entails a loss of) air (circulation and light), but not to a thick wall, which diminishes the usable area in his Chatzer. The Mishnah teaches, this is not so.
QUESTIONS AGAINST THE OPINION THAT HEZEK RE'IYAH IS CONSIDERED DAMAGE
Question #1 (Mishnah): And similarly in a garden...
Answer: A garden is different, for R. Aba's reason.
(R. Aba): One may not look at another's field when it is laden with crops (due to Ayin ha'Ra'ah, i.e. jealousy, which arouses harsh judgment from Heaven)).
Question: It says 'similarly'! (This implies that it is for the same reason.)
Answer: This refers to (area needed for each of) the building materials.
Question #2 (Mishnah): If the wall of a Chatzer fell, they must rebuild it to four Amos.
Answer: A wall that fell is different (Rashi - because they already agreed to have a wall; Tosfos - because they are used to doing things in the Chatzer that require privacy).
Question: (The answer is obvious.) Why didn't the one who asked anticipate this answer?
Answer: He thought that the law is the same even if there was never a wall there, and the case was taught due to the Seifa;
(Seifa): (Even if the wall was originally more than four Amos,) we do not force him to rebuild it more than four Amos.
Question #3 (Mishnah): We force partners in a Chatzer to build a Beis Sha'ar (a wall to stop people from looking in through the gate) and door for the Chatzer.
This teaches that Hezek Re'iyah is considered damage!
Answer: No, that only shows that Hezek Re'iyah to the masses (who walk nearby) is damage.
Question #4 (Mishnah): We split a yard only if each partner will receive four Amos.
Suggestion: If each will receive four Amos, they divide it with a wall!
Answer: No, they make a simple divider in between.
Question #5 (Mishnah): If one wants to build a wall opposite his neighbor's window, the wall must be at least four Amos above or below the window, and (if it is higher) at least four Amos away from the window.
(Beraisa): We require four Amos above, lest he crouch down from his wall and look into the window. We require four Amos below, lest he stand on his wall and see through the windows. We require four Amos away, so he will not block light.
Answer: We are more stringent about Hezek Re'iyah in a house.
Question #6 (Rav Nachman): If Reuven's roof overlooks Shimon's yard, he must build a four Amos wall on his roof. (This shows that Hezek Re'iyah is damage!)
Rejection: That case is different. Shimon constantly needs to use his yard. He does not know when he must beware, for Reuven goes on his roof at irregular times.