This Mishnah discusses a wall built for privacy between two neighbors who wanted to divide their front yard. We'll talk about what happens if that wall falls down. If there was a wall dividing a courtyard between two different neighbors, and that wall fell down, we pressure and force each neighbor to pay half the cost of rebuilding it, up to a maximum of four amot (cubits), the height needed to maintain privacy.
One neighbor will take charge of the project, handle all the arrangements, and require the other neighbor to pay half the cost. If the wall is being built, we assume that both people paid: not only the neighbor handling the job, but the other neighbor as well. Everyone knows the halakha (Jewish law) that when two neighbors are building a shared wall, both have to contribute. So if one neighbor went ahead with the building process, we assume that the other person contributed his half of the money as well to get the job started. Otherwise, the person wouldn't have built it. He wouldn't have started the job without getting paid.
So if the neighbor who was in charge of the building comes afterward and says, “Oh, I never got paid for the job,” and the other neighbor, who was only paying and wasn't handling the building, says, “Of course I paid,” we believe the second neighbor when he claims that he already paid. We say that the first neighbor wouldn't have started the process or the job if he didn't have the money to start it.
If the original wall was taller than four amot and then fell, we don't require either neighbor to pay for more than four amot. Even if the original wall was taller, we allow them to pay only the minimum cost of rebuilding it up to four amot.
Let's say one of the neighbors decided to pay out of pocket for more than four amot. Both neighbors contributed to the supplies and materials for up to four amot. Then one of the neighbors, on his own, wanted to increase the original height to more than four amot because the original wall was taller. He wanted to restore it to its original height. But he can't force his other neighbor to contribute. He wants to go higher than four amot, so he is going to pay for it.
But let's say a little later, the neighbor who only wanted to pay for the minimum—a four-amot wall—built another wall right near the original one, to the same height, and wanted to put a roof on it to connect the two walls. What do we see retroactively? Even if he hasn't put the roof on yet, to connect the two, we impose on this neighbor the obligation to pay half for the first wall's section above four amot, whatever height it was built to. The fact that he built a second wall parallel to the first shows that he really wanted and benefited from the original wall being rebuilt to its original height, more than four amot.
Since he's benefiting from it, putting another wall near it, and planning to put a roof on top of them, he really wanted the original wall to go back to its original height. He was just trying to get out of paying the extra money. He wanted to pay the minimum for a four-amot wall. Now that he has built the second wall to reach the same height, we see that he really wants it and benefits from it, and therefore we'll require him to pay for that section above four amot as well.
But unlike in the first case of the Mishnah, we can assume that he did not pay for this upper portion of the wall unless he brings proof that he already paid. In this situation, one of the neighbors is paying to rebuild the dividing wall because he may have wanted it. Later, when we see the other neighbor building the parallel wall, we tell him, “Okay, you now have to pay retroactively for the portion of the first wall that went all the way up to the original height, because you didn't pay originally, since you weren't obligated to.” And so the neighbor who built it decided to take all the responsibility upon himself.
If the neighbor here claims, “Oh, I already paid. I already paid originally,” we don't believe him, because he wasn't obligated to pay until he built that second wall. So we force that second neighbor to pay for the upper portion of the original wall, which he only later showed that he really wanted and benefited from.