The gemara reports a case in which Reuven's fields surrond Shimon's on three sides. If Reuven erects fences on three sides of Shimon's property then Shimon is not obligated to contribute to the costs incurred by Reuven. If REuven erects a fence on the fourth side, however, Shimon becomes obligated to help defray the costs of all four fences. Why?? What is the difference here??
Joel Morris, Winnipeg, Manitoba, Canada
The simple difference in the Gemara is that Shimon was not helped much by Reuven's barriers, as they did not close off his field from being invaded by animals (and the like). However, once his field is closed off from all four sides, he is now benefitting from an enclosed field (see Rashi DH "Mai Ahani Ley").
If you are bothered by the fact that Shimon has to pay because Reuven decided he wanted a fence, you are in good company. The Rashba asks this question in Bava Basra (4b), and answers in the name of the Ramban that he only has to pay if he does not protest that he is not interested in paying for the project. If he does, he is exempt from paying.
All the best,
Yaakov Montrose