Bava Kamma 6B

שור יוכיח!

The Talmud suggests: The primary category of Ox will prove that this fact, that the ox, i.e., the cause of the damage, entered the public domain with permission, is not a reason to exempt its owner from payment, as that is the nature of the primary category of Ox, and one is liable for damage caused by his ox.

מה לשור שכן דרכו לילך ולהזיק.

The Talmud questions the derivation from Ox: What is notable about Ox? It is notable in that it is the typical manner of the animals in these categories to proceed from one place to another and cause damage. By contrast, in the case of the sewage, it is not its typical manner to proceed and cause damage. Accordingly, how can one learn the halakha in this case from the primary category of Ox?

בור תוכיח! וחזר הדין.

The Talmud suggests: If one attempts to challenge the derivation from Ox in this manner, it is possible to respond: The halakha of the primary category of Pit will prove that the lack of this characteristic, of the typical manner being to proceed and cause damage, is not a reason to exempt one from paying restitution for the damage, as that is the nature of Pit, and one is liable for damage caused by his pit. The Talmud notes: And as Pit and Ox each prove that the defining characteristic of the other is not decisive in establishing liability or exemption, the derivation has reverted to its starting point, and liability in these cases is derived from the common denominator of Pit and Ox. This is the halakha added by the common denominator stated in the Mishnah.