“Ha-motzi me-havero, alav ha-ra’ayah” – or, in more colloquial English, “Possession is nine-tenths of the law.” According to the halakha, if you want to make a claim on someone else, you will have to prove that you are right. This is the underlying message of our Mishna.
One example of this rule in our Mishna is a case where one ox was chasing another, and the second ox became injured. The owner of the injured animal claims that the injury was caused by the first ox; the owner of that animal claims that the second ox became injured on a rock. Ha-motzi me-havero, alav ha-ra’ayah – we will only make the owner of the first animal pay if the second one can bring witnesses that the injury was done by the first ox.
From Rashi’s explanation of this case it appears that the claim that the second ox’s injury came from a rock means that the owner of the first animal denies any involvement in the accident, arguing that the injured animal simply rubbed against a rock and hurt itself. If, however, the animal became injured because it was running away from the first ox, the owner of the first ox would have some level of responsibility for the injury. The Rashba disagrees and says that the owner of the first ox will only be held responsible if his ox actually gored the second one, but if the second ox became injured on a rock while running away from the first ox, the owner would not be responsible. This is because the ox would only be considered a case of gerama be-nezikin – a distant source of damage, not a direct one, and gerama be-nezikin is patur (he is not responsible).