First, a long mishnah - paying damages for the ox that damages. Depending on where the damage took place. But what impact does the location have, once you're paying full damages? The public domain is less strict for common damages, and more strict for "keren," the less expected damage. But there are limits - where the derived law is the same as the sourced law. Plus the application of the logical tool of "da-yo" - the fundamental position that the logic of a kal va-chomer can only be applied as far as it can (with whatever dispute), and not to apply to logic without limitation. On this daf, the logic is explored in the context of ritual impurity.