Bava Batra, Chapter One, Mishnah Three
Mishnah three discusses a person who builds a fence partially around another persons property and wishes that person to share in the costs of the fence.
1) If a mans land surrounded his fellows land on three sides, and he fenced it on the first and the second and the third sides, they do not obligate him [to share in the costs].
a) Rabbi Yose says: If he rose a built a fence on the fourth side, they obligate him to share in all of the costs.
In the scenario discussed in this mishnah Reuven owns three fields which surround Shimons fields on three of its four sides. If Reuven were to fence in all three of his fields he would have built a fence around three sides of Shimons fields. Since this fence does not totally surround Shimons field, Reuven cannot claim that Shimon benefited from the field and should therefore share in the costs. Rabbi Yose adds that if Shimon were to fence in four sides, he would thereby show that he wished to have his field fenced in. In that case he is obligated to share in the costs of all of the fences, since he now benefits from them all.
Questions for Further Thought:
· If Reuven owns fields surrounding all four sides of Shimons field and he puts a fence around all of them, will Shimon have to share in the costs?