Bava Batra 35 - Let He Who Is Stronger Prevail
In a case where ownership of a property is disputed - for example, one side says, "It belonged to my forefathers" and the other side says, "It belonged to my forefathers," but neither litigant can prove his claim - the court withdraws and allows the litigants to fight over possession of the property.
Once one gets the object, can the other one take it back? Some say "no" - because the court would not allow a perpetual feud. Others say that "yes, he can take it back."
One rational for this principle is that the true owner would eventually come up with the proof of his claim. Furthermore, the true owner would strive more vigorously to retain his property, while the thief would not, knowing that his victory may be short-lived, because the owner may subsequently prove his claim.