If a neighbor is surrounded on three sides by one land owner and that land owner put up 3 walls, the inner neighbor doesn’t need to pay. But if a fourth wall is put up, he does. There is a debate about whether it matters who put up the fourth wall and whether or not the inner neighbor needs to share the costs of just the 4th wall or all the walls. Or whether he needs to pay according to the actual cost of the wall or only according to the cheapest market rate for a wall or possibly even only for what it would cost to hire a watchman to protect the field. A story is brought about Ravina and Runia who were neighbors and got into a court case about this as Runia didn’t want to share in the costs. If a wall between 2 neighbors falls, even if the wall was higher to begin with, one need only chip in to build a four cubit high wall. However if one builds an inner walls and plans to attach a roof from the high was to the new inner wall, one shows that the high wall serves his needs and needs to share the cost. The mishna makes assumptions about in which situations we can assume the neighbor has already paid and in which not. The gemara attempts to connect this with assumptions about one was can be believed that one returned a loan if the lender says he/she did not. The gemara rejects the connection between the cases.