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 method is that of a kal va-homer, the first principle taught by Rabbi Yishmael. The method of kal va-homer – usually translated as an A fortiori argument – allows us to learn one law from another by arguing that if the less stringent law included a stringency, we can conclude that the stricter law includes that stringency, as well.
Although the method of kal va-homer is considered to be a powerful one, it is limited in cases where there is an attempt to derive more than the original law included, as the Gemara tries to do on our daf by suggesting that we can learn that an ordinary thief is punished by paying back double by means of a kal va-homer from the case of a shomer (a guard) who claimed that the object he was watching was stolen from him. 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 (earlier in Bava Kamma, on daf 25) explained that the concept of kal va-homer – and dayyo – stem from the story of Miriam who spoke inappropriately about her brother Moshe (see Bamidbar chapter 12). As punishment, she was struck with tzara’at, 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.