2) BUILDING A FENCE AROUND A NEIGHBOR'S PROPERTY
QUESTIONS: In order to prove whether or not one must pay for the benefits he gains in a case of "Zeh Neheneh v'Zeh Lo Chaser," the Gemara quotes a Mishnah (Bava Basra 4b) in which Rebbi Yosi and the Chachamim argue with regard to a landowner (Reuven) who owns the fields surrounding the property of another landowner (Shimon) and who builds a fence around his fields. The Chachamim rule that if Reuven builds a fence around three sides of his property where his property borders with Shimon's property in the center, and he ends up enclosing three sides of Shimon's property, Shimon does not have to pay Reuven for any part of the cost of the fences. This implies that if Reuven builds fences around his property so that it entirely encloses Shimon's property (such as when Shimon's property is totally surrounded by Reuven's), Shimon should have to pay Reuven even though Reuven does not lose anything because of Shimon (since Reuven would have had to build the fence around his property in any case). Rebbi Yosi argues and says only when Shimon builds the fence on the fourth side (thus completing the enclosure around his property) must he pay Reuven for part of the cost of all the other fences that Reuven built on the other sides. If Reuven built all four fences, Shimon does not have to pay him anything.

The Gemara proves from the Chachamim's ruling that one is Chayav in a case of "Zeh Neheneh v'Zeh Lo Chaser" (that is, the beneficiary must pay for the benefit he receives when the benefactor loses nothing). The Gemara then rejects the proof by saying that in this case Reuven indeed loses something (and it is a case of "Zeh Neheneh v'Zeh Chaser"), since he can claim that Shimon caused him to build a larger fence than he would have needed to build had Shimon not been there.

The Gemara then proves from Rebbi Yosi's ruling that one is exempt in a case of "Zeh Neheneh v'Zeh Lo Chaser." The Gemara rejects the proof by saying that the reason why Rebbi Yosi exempts Shimon from sharing the costs of the fence is that Shimon may claim that a strong wall provides him with nothing more than he could have gained with a very inexpensive fence made of thorns.

RASHI (DH Ha Revi'is, and DH Ha Makif) explains that the Chachamim's ruling was stated in a case in which Reuven builds a fence around three sides of Shimon's field, and Reuven owns land around only those three sides. When the Gemara infers that if Reuven builds four walls around Shimon, Shimon must pay Reuven for part of the cost, the Gemara refers to a case in which Reuven bought the property along the fourth side of Shimon's field and then built a fence there, completely enclosing Shimon's field. In such a case, Shimon is obligated to pay part of the cost of the enclosure.

Why does Rashi not explain even in the case in which Reuven builds three fences around Shimon's field that Reuven owns the field on the fourth side but chose not to enclose it with a fence? Perhaps Rashi bases his explanation on an inference from the wording of the Mishnah in Bava Basra which says "ha'Makif Chaveiro mi'Shalosh Ruchosav," implying that Reuven owns land around Shimon's field only on three sides and not on the fourth. However, now the question may be asked on the Mishnah there: Why does the Mishnah explain that Reuven's fields surrounded Shimon's only on three sides and not on the fourth?

(b) Rashi (DH mi'Shalosh Ruchosav, and DH Ha Revi'is) explains that the fences on the three or four sides were built on the inner side of Reuven's fields where they border on Shimon's property. Besides these fences, there was another set of external walls around Reuven's fields which separated Reuven's property from the neighbors' property on the side opposite from Shimon.

If Reuven already has external walls around all of his fields that surround Shimon's property, what does Shimon gain by the internal fence? He is already protected from all sides from Reuven's external wall! Moreover, why did Reuven build those internal fences in the first place if he is already protected from outside infiltrators? If he built the fence in order to separate between his fields and Shimon's field, it is obvious that Shimon should share the cost of the fences if he benefits from them; since Reuven clearly built the extra fences only because of the presence of Shimon's field, Shimon's presence clearly did cause Reuven a loss (as the Gemara concludes, Reuven may claim that Shimon's presence caused him to build a longer fence). Why, then, does the Gemara initially suggest that this is a case of "Zeh Neheneh v'Zeh Lo Chaser," if Reuven does lose as a result of Shimon? (TOSFOS DH At Garamt; see NACHALAS DAVID to Bava Basra 4a.)

Tosfos and other Rishonim indeed disagree with Rashi and explain that the Gemara is discussing only the external fences of Reuven's fields (and there are no internal fences).

(c) If Rashi chooses to explain that the fences under discussion separate between Reuven's fields and Shimon's field, why does he mention at all that there is a separate set of external fences which separate between Reuven's fields and his other neighbors? Perhaps there is only one set of fences -- the set about which Rebbi Yosi and the Chachamim argue!

ANSWERS:
(a) The reason why the Mishnah in Bava Basra explains that Reuven does not own the field on the fourth side of Shimon's land in the case where he builds three walls is that if Reuven owns the fourth side as well, then even he does not benefit from the fences he builds around Shimon's field, since there is still a breach on the fourth side. Accordingly, it is obvious that Shimon would not have to share in Reuven's expenses, since Reuven's fence-building was purposeless. The only situation in which Shimon might be obligated to share the expenses is where Reuven benefits from the fences by his fields being completely enclosed -- which is the case when Reuven owns only the property bordering three sides of Shimon's field and not the property on the fourth side. (The diagram printed in the Vilna Shas is incorrect. The two edges of Reuven's field in the south should also be enclosed by the fence and not left open.) The Mishnah teaches that Shimon nevertheless does not have to contribute to the costs of the fence since Shimon himself is not fully enclosed by it.

(b) Rashi maintains that as long as the person who derives benefit does no action to take something from the person who is Chaser but merely indirectly causes an expense to the other person, it is not called a case of "Zeh Neheneh v'Zeh Chaser." For example, in the case of the person who occupies someone else's vacant house without the owner's consent, where the owner otherwise would have rented out his house for money, the occupant is causing the owner a loss through his action of moving into and living in the house. In the case of the outer fields surrounding the inner field, although Reuven builds a fence between his fields and Shimon's field only to prevent Shimon from entering his field or to mark the borders of his fields, the Gemara initially assumes that this is not considered a loss to Reuven since Shimon is not doing any action. (Indeed, he even might have owned his field before Reuven purchased the surrounding fields. Even if Shimon purchased his field afterwards, his purchase or presence does not constitute an act of causing a loss to Reuven.)

Moreover, Shimon can claim that Reuven did not build the fence because of a fear that he would steal from Reuven, but because Reuven wanted to make the border of his field clear to any potential purchasers. Hence, Shimon is not involved in causing any expense to Reuven.

This explains how Reuven and Shimon both can benefit from the inner fence while, nevertheless, it is considered "Zeh Neheneh v'Zeh Lo Chaser."

The Gemara concludes that it is considered "Zeh Neheneh v'Zeh Chaser" since Reuven built the fence only because of Shimon's presence, even though Shimon did not do any action to cause Reuven's loss, and even though Reuven built the fence only to delineate the boundary of his field (and not out of fear that Shimon would trespass).

(c) Rashi may have inferred that there were external fences around Reuven's property from the wording of the Gemara which refers to the inner fences as "extra fences." These words imply that there already exists some fence that Reuven had to build without Shimon, and the inner fences were built in addition to that fence. (See Tosfos, DH At Garamt.)

The question now, however, is why does the Gemara assume that Reuven built external fences besides the inner ones? The answer might be that if Reuven had built only inner fences, it would be obvious that Shimon cannot exempt himself from sharing the expenses because of "Zeh Neheneh v'Zeh Lo Chaser." Reuven would certainly be considered to be losing something due to Shimon, since Reuven's actions showed that he found it necessary to build the fence there only because of Shimon's presence, but on all other sides of his fields he did not find it necessary to build a fence (as pointed out above in the answer to the second question). Therefore, the Gemara -- which tries to prove from the Chachamim's ruling that one is Chayav in a case of "Zeh Neheneh v'Zeh Lo Chaser" -- must have understood that Reuven built fences around the external boundaries of his property as well as along the border with Shimon's property.