In the Mishnah on today’s daf (=page) Rabbi Me’ir attempts to derive a halakhah about the ritual status of a tereifah – an animal that had an injury that would cause its death – that was discovered after being killed by means of melikah (see above, daf 64), by means of a kal va-homer (usually translated as an a fortiori argument) from ordinary kosher slaughter.Rabbi Yossi objects, claiming that the kal va-homer would not extend to the case of melikah, which is an extrapolation beyond the parallel case of shehitah.
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 halakhah from a kal va-homer, but not more than the original law itself.
The Gemara quotes a baraita that explains that the concept of kal va-homer – and dayyo – are biblical in origin. They stem from the story of Miriam who spoke inappropriately about her brother Moshe (see Bamidbar 12). As punishment, she was struck with tzara’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 14 days, dayyo limits the punishment to the same amount of time that she would have been embarrassed by her father.
One question raised by the rishonim is why logic would lead us to conclude that Miriam should have been banished for 14 days. Why not 8 days? Or forever?
Rabbenua Tam is quoted as connecting this with the idea that there are three partners in the creation of a person – his mother, his father and God. Thus God is the equivalent of both mother and father and offense against Him deserves double banishment.
Rabbenu Hayyim ha-Cohen suggests that Miriam deserved just a little extra banishment, but the minimum time that someone suffering from tzara’at is banished is a week, so any additional banishment must be for a full extra week.