Although many of the laws of nezikin are presented in the Torah, there are others that do not appear in the Torah at all and must be derived by the sages using methods of hermeneutics like the 13 rules of interpretation presented by Rabbi Yishmael. One case that does not appear in the Torah is a case of keren be-reshut hanizak: an ox that enters a private domain and causes damage by goring or similar actions. It is clear that the owner will be held responsible to pay for the damage, a rule that is supported by the application of the first rule – kal va-homer (which is usually equated to the Argumentum a fortiori in classical rhetoric). The Tanna Kamma rules that in this case the owner will pay half-damages; Rabbi Tarfon rules that he will have to pay full damages.
Rabbi Tarfon explains his position by presenting a kal va-homer that works as follows:
- In cases of shen and regel (damage caused by an animal that is eating or walking normally) in the public domain, the owner does not have to pay at all, although if the damage took place in a private place, the owner will have to make full restitution.
- In cases of keren (damage done by an animal that gores) there is a greater level of responsibility in the public domain – the owner will have to pay half-damages. We can therefore conclude that in a private place there will also be a greater level of responsibility, and the owner will have to pay full damages.
While the Tanna Kamma accepts the kal va-homer to the extent that we must conclude that the owner is responsible for damage done by his animal in a private place, but he rejects Rabbi Tarfon’s conclusion, arguing that we cannot hold him more responsible that he was in the primary case. Thus the Tanna Kamma rules that the owner will pay half damage, just like he does in the public domain.
Limiting the conclusions that can be reached by means of a kal va-homer in this manner is called dayyo – “enough.” It is enough to learn a parallel halakha from a kal va-homer, but not more than the original law itself.
The Gemara explains that the concept of kal va-homer – and dayyo – stem from the story of Miriam who spoke inappropriately about her brother Moshe (see Bamidbar 12). As punishment, she was struck with tzora’at (biblical leprosy), and was forced to leave the encampment for seven days. The Torah explains that had her father banished her, surely she would have been embarrassed for seven days – now that she was banished by God, she will have to be removed for that length of time. Although logically banishment because of God’s anger should have lasted twice as long, dayyo limits the punishment to the same amount of time that she would have been embarrassed by her father.