Bava Basra, Perek 1, Mishnah 4. In the first Mishnah of the chapter, we dealt with a wall built to provide privacy and prevent damage caused by looking, and afterward we digressed from the topic for a few Mishnayos. Now we return to it.
"Kosel chatzer shenafal":
This refers to a courtyard where a wall was built to provide privacy and prevent damage caused by looking - so that a person cannot see his fellow's private affairs - and this wall fell. The Mishnah says: "Mechayavin oso livnoso ad arba amos" - they obligate him to build it up to four amos. Each of the neighbors in the courtyard can force his fellow to rebuild the wall to a height of four amos. Four amos (approximately six feet) is the height that prevents people from peeking over the wall. Any addition beyond this height does not provide additional privacy - it may have other benefits, but not privacy - and therefore a person is not permitted to demand it from his neighbor. However, up to four amos, he has the power to force him to build.
"B'chezkas shenasan, ad sheyavi re'ayah shelo nasan":
In a case where after the building of the wall, the neighbor who built it claims that his fellow - let us call him the stingy neighbor - did not pay his share, the Mishnah rules that this claim is not believed. Until the initiating neighbor brings proof that the stingy neighbor did not share in the expenses, his words are not accepted. The reason for this: it is not at all logical that a person who knows he can force his neighbor to pay half the amount would first build the wall and only try to collect the money afterward. If he could have collected the money in advance, he would not have spent it out of his own pocket; and if the neighbor had acted stingily, he would have brought him to beis din. Therefore, when they come to beis din, the claim that the payment was not made is not believed; rather, we assume it was paid even before the start of construction.
What would be considered proof that he did not pay?
It is difficult to bring proof about something that did not happen; one can only bring proof about something that did happen. But we can imagine a case: the initiating neighbor went on the day of construction accompanied by two of his friends, who serve as witnesses, and said to his neighbor: "We are building the wall today, and you must give half the money," and the other replied: "I will not do so." In such a situation, the initiating neighbor will have witnesses testifying that the stingy neighbor did not give his share.
"Me'arba amos u'lema'alah ein mechayavin oso":
If the initiating neighbor says: "I do not want a wall just for privacy, but rather I want a ten-foot wall, because then I can also install a basketball hoop on it and the like" - he does not have the power to obligate his neighbor to bear half the expenses except up to a height of four amos.
"Samach lo kosel acher":
If the stingy neighbor, after the wall is standing at a height of ten feet, builds a parallel wall next to it at a height of ten feet - for example, for parking his car - it shows his intention is to use the dividing wall between the two properties together with the new wall next to it to build a new structure. In such a case, even if he has not yet placed a roof connecting the parallel wall to the dividing wall, we already obligate the stingy neighbor to pay half the expense from four amos up to ten feet, since it is clear that he wants to benefit from a ten-foot wall.
And here the pendulum swings in the opposite direction: if after the stingy neighbor built his parallel wall, the initiating neighbor brings him to beis din and claims, "He is using the ten-foot wall, and he must pay for half of a ten-foot wall, not just half of a four-amah wall" - and the neighbor claims he already paid, he is not believed, because it is not logical. Since they reached a dispute, it is clear that out of stinginess he is trying to evade payment and use the extra height. Therefore, he must bring proof that he indeed paid the addition from four amos up to ten feet, or else he will be obligated to pay.
In summary: In this Mishnah we returned to the matter of the courtyard wall built to prevent damage caused by sight. We learned that if the wall falls, either neighbor can compel the other to rebuild it up to four amos; that when a dispute arises over payment, the neighbor is presumed to have paid, until proof is brought that he did not pay; that from four amos and above we do not obligate him, since this addition is not for the sake of privacy; whereas when he builds another wall next to it and reveals his intent that he wants the additional height - the presumption is reversed, and he must bear the expenses of the addition as well, unless he brings proof that he paid.