Bava Basra, Chapter 1, Mishnah 3. The Mishnah before us deals with a situation different from the previous ones: we are not discussing a wall here, but rather a fence around fields.
In the first two Mishnayos we dealt with walls:
The wall in Mishnah 1 - was intended for privacy.
The wall in Mishnah 2 - was intended to prevent the evil eye, so that people would not peek inside.
Therefore, these two walls were four amos high, so that no one could peek over them. In contrast, the fence before us is only intended to protect against animals entering the field and the like, and therefore ten tefachim is sufficient - a much lower wall.
The Scenario of the Mishnah:
What is the halachah when one of the neighbors is not interested in a fence around his field or does not want to bear its cost, while the second neighbor is interested in it? The scenario is as follows: one person is called the nikaf - the surrounded one - and the fields around him belong to his neighbor. We can compare this to a checkerboard or chessboard: the nikaf holds the white square, and around him, on all four sides, are the black squares of the neighbor. Let us assume that all four black squares belong to the neighbor - the makif (the surrounder) - and he wants to build fences around his small fields.
If he built fences around one, two, or three fields, the nikaf in the middle is left open on the fourth side, and he derives no benefit from this at all, since animals can enter from that side. Therefore, he cannot be obligated to participate in the cost of building the fence around the grain field, even though it is customary to build fences, because he can claim: I do not benefit from this fencing, since I am open on the fourth side.
The Text of the Mishnah:
"Hamakif es chaveiro mishalosh ruchosav" - a person who surrounds his friend with his fields on three sides, like in the chessboard scenario, and closes in on him. "Vegadar es harishonah ve'es hashniyah ve'es hashlishis" - the owner of the black squares builds a fence around three of the sides. In this manner - "ein mechayevin oso": even though he spent money to build the fence, he cannot compel the nikaf in the middle to participate in the cost of the fences, because the latter derives no benefit from them at all, as he is open on the fourth side.
The Opinion of Rabbi Yosi:
These words are the opinion of the Tanna Kamma. However, Rabbi Yosi says: "Im amad vegadar es harevi'is - megalgelin alav es hakol" - once a fence is also built on the fourth side, he can be obligated to pay for everything.
Who Built the Fourth Side?
The wording of the Mishnah is not explicit regarding who builds the fourth fence - the surrounder or the surrounded, the owner of the black squares or the owner of the white square in the middle - and therefore the Gemara brings both possibilities.
The first opinion, which is also cited by the Bartenura: Even if the surrounder, the owner of the black squares, is the one who built the fourth fence - and ostensibly built it to enclose his own small fields - the inevitable result is that the surrounded person in the middle is now fenced in on all sides. Therefore, "Im amad" - if the surrounder went ahead and covered the fourth side, "Megalgelin alav es hakol" - we roll everything onto him: the owner of the black fields may compel the surrounded person in the middle to pay for half the cost of the fence around his field, since ultimately, he now benefits from it.
All the more so, via a kal vachomer, if the surrounded person himself - the owner of the white square - is the one who built the fourth wall.
The Tanna Kamma disagrees: As long as the surrounded person did not build the fourth wall himself, we do not compel him, since he did not demonstrate that he wants a fence.
The Halachah:
The halachah follows Rabbi Yosi: From the moment the surrounded person in the middle benefits from the fence and is no longer exposed, he is obligated to bear half the cost of the fence, according to the standard rate for that time and place. Therefore, he will be required to share the cost with his surrounding neighbor, since he derived a benefit from it.
In summary: In this Mishnah we learned about a fence ten tefachim high, which is intended to prevent animals from entering, unlike the walls in the previous Mishnayos. As long as the surrounded person is exposed on one side, we do not obligate him to share in the cost, since he does not benefit. However, once the fourth side is also fenced in, whether by the surrounder or by the surrounded person, the surrounded person benefits from the fence - and the halachah follows Rabbi Yosi, that we roll everything onto him.