In the last Mishnah, we spoke about how a grain field would not normally have walls put up for privacy, but there were short walls to keep animals from destroying all the crops. In this Mishnah, we'll talk about those types of walls. The Mishnah says that if somebody's grain field surrounds his friend's grain field on three sides—in other words, he buys and owns the fields on three sides of his friend's grain field—then his friend has the inner grain field and is fully surrounded by his friend's grain field on three sides. We'll call the man with the three fields that surround him Reuven, and we'll call the man who has the field on the inside, surrounded on three sides, Shimon.
Reuven wants to build walls to divide up his property and Shimon's, and the point of these walls would also be to keep animals from coming in. So he builds on three sides, one next to each of his fields. Shimon would be entrapped, or he would now have walls on three sides, but one side would still be open. Shimon's inner field is now surrounded by three sides of wall, but one side is open. So the question is: could Reuven, who's building these walls, force Shimon to contribute? We do not require the owner of the inner field, Shimon, to help pay for these walls because, bottom line, he doesn't have complete benefit. If one wall is open, animals can get in. They can get into his field and damage it through that fourth side. Therefore, he doesn't have a real benefit, and Reuven can't obligate Shimon to pay.
The implication is that if Shimon, who's surrounded on three sides, does want to close up that fourth side, he then reveals that he really likes and benefits from the other three walls. So if he does that, he would be required to pay Reuven for building three components of his fence when he built the fourth, as we'll discuss momentarily. He is showing that he likes it and wants it. Rabbi Yosi says that if Shimon, the man with the inner field, goes and builds on that fourth side, the field is now fully enclosed, and we put the entire expense on the owner, Shimon. We put the half payment on him. The fourth side is also becoming enclosed. He is now revealing that he's benefiting from these walls. Animals can't get in any longer, and therefore we require him to pay for half the cost of these four walls.
You may say that Reuven seems to be the same as Tanna Kamma (first tanna). As we just implied above, according to Tanna Kamma as well, if a person adds that fourth wall, he'd be obligated to pay because he's showing he gets benefit. And then we seem to be repeating the same thing with Rabbi Yosi: if he gets up and builds that fourth wall, he has to contribute up to half of that wall. So the real halachic (pertaining to Jewish law) difference between the two is that according to the first Tanna Kamma, the owner of the inner field only has to pay half the cost—he has to partner up in the cost—but based on the most inexpensive type of fence out there in the market, even one made of reeds. Even if the actual wall was built from something much sturdier, like expensive stone, the inner Shimon, who has the inner farm, would only be obligated to pay for half the minimum fence, because he's only building it, he wants a cheap fence to keep the animals out.
Reuven, on the other hand, who said he's paying for half—what did he mean by that? That he has to pay for half of the entire expense, no matter what it is: half of the cost of the actual walls, what it costs to build. Therefore, even if it was a more expensive material, he would have to pay half. They disagree on the minimum somebody would have to pay.