Bava Basra, Chapter 1, Mishnah 2. The Mishnah continues to discuss dividing property and erecting walls between neighbors. Here, however, we are not dealing with the damage of being seen and the invasion of privacy, but rather with a wall intended to prevent the evil eye - a situation where a person stares at his neighbor's plot and is jealous of what is happening there.
The context is building a wall around a garden, meaning a vegetable patch. The concern is that the neighbor will look at the vegetables and say: "Look how beautifully his eggplants are growing," thereby causing damage through the evil eye. This is not everyone's custom, but it is not unreasonable either: in many places it was accepted practice to build a wall around the vegetable patch, so that the neighbor would not peek inside and cause this damage.
"Vechen baginah, makom shenahagu ligdor - mechayevin oso" - and similarly in a garden, a place where it is customary to build a wall, we obligate him:
The same principle of dividing the area and allocating resources to build a wall applies here as well. In a place where it is customary to build a wall around the garden, the neighbors' demand is within the realm of reason - reasonable people request a wall so that strange eyes will not peek inside. Therefore, "mechayevin oso" - we obligate him: one neighbor can compel his fellow to allocate half of the land, and bear half of the expenses, in order to build a wall around both of their vegetable patches.
"Aval babikah, makom shenahagu shelo ligdor" - but in a valley, a place where it is customary not to build a wall:
The word bikah usually means a valley, but here it refers to a place where grain is grown, meaning a field, as opposed to a vegetable garden. The Mishnah means that this is the kind of place where people do not usually insist on building a wall around a grain field. A fence to keep animals away is one thing, but an actual wall, four amos high, is not a regular occurrence there. Therefore, "ein mechayevin oso" - we do not obligate him, and a person cannot compel his neighbor to bear half the cost of a wall around his field.
Tosafos explains the difference between a vegetable garden and a grain field:
In a garden: The crops grow throughout the year - if it is not the eggplant, then it is the tomatoes or the cucumbers, and so on. The produce is always present for onlookers to see.
In a valley: For most of the year there is nothing ripe in the field, except for about one month a year when the grain stands outside and arouses the neighbor's jealousy. This period of time is not enough to justify the immense cost of building a tall wall around a large field.
One who wishes to build a wall in a valley:
Although he cannot compel his neighbor to bear half the cost, he has permission to build it himself, provided that he does these two things:
"Kones lesoch shelo uvoneh" - he must withdraw slightly into his own property, so that the entire wall stands on his plot alone, without crossing the boundary line and without encroaching at all onto his neighbor's property.
"Ve'oseh chazis mibachutz" - he must make a distinguishing mark, a square of plaster about an amah by an amah, on the outer side of the wall, which is the side facing the neighbor. This mark proves that the wall is his, since it was placed where everyone can see it.
Why specifically on the neighbor's side and not on his own side? Because if the neighbor does not see what is being done, the builder could make the mark after the fact without his knowledge, and claim that the entire wall is his even in a case where both shared the expenses. But when the mark is made exclusively on the neighbor's side, it is clear to all that he is the one who made it and took the full cost upon himself. We are not concerned that the neighbor might peel off the plaster, since traces of what was there will still remain, and everyone will know that he is a swindler.
"Lefichach im nafal hakosel - hamakom veha'avanim shelo" - the builder takes all the stones for use as building materials, along with the entire space under the wall, and he does not share anything with his neighbor, since he proved with the mark on the outside that it all belongs to him.
"Im asu mida'asan":
What is the halachah when the two neighbors agree between themselves to build a wall between their grain fields? In this case, "bonin es hakosel ba'emtza" - they revert to the halachah of building a wall on the property line, and each one contributes half of the space. "Ve'osin chazis mikan umikan" - they make a mark on both sides of the wall, so that each one will have a sign testifying that he bore half the cost.
"Lefichach im nafal hakosel - hamakom veha'avanim shel sheneihem" - it does not matter in which direction the stones fell, since there is clear evidence here: given that both of them made a mark on their respective sides, confirming that they paid equally to build the wall, they are obligated to share the space and the building materials equally.
In summary: In this Mishnah we learned about a wall built to prevent ayin hara. In a garden, where crops are present all year round, in a place where it is customary to build a fence, we compel the neighbor to participate in providing the land and the expenses. In a valley, where the crops only stand for a short time during the year, in a place where it is customary not to build a fence, we do not compel him; and one who wishes to build must recede into his own property and make a mark on the outside, and therefore the space and the stones are his alone. However, when they build it with mutual agreement, they build in the middle and make a mark on both sides, and therefore the space and the stones belong to both of them.