We learned in the mishnah on yesterday’s daf that:
One who says to his wife: “You are hereby to me like my mother,” they encourage him to dissolve the vow on other grounds so that he will not (make such vows) lightly.
The man’s vow implies he will not have sex with his wife, just as he does not have sex with his mother. The mishnah requires this man to formally dissolve the vow, and says he must do so in order that he not take vows lightly — that is, so he will not make them willy nilly.
Today’s daf tries to understand exactly why the man must formally dissolve this vow.
We already learned on Nedarim 12 that if one makes a vow restricting something with reference to something forbidden by Torah law, it doesn’t take effect at all. Since incest is forbidden by Torah law, this vow which uses the language of comparison to incest is ineffective. So it seems that he is dissolving something that doesn’t exist. Indeed, the Gemara today quotes a beraita that states this explicitly:
“You are hereby to me like the flesh of my mother, like the flesh of my sister, like the fruit of a tree during the first three years after its planting, or like diverse kinds in a vineyard …” — he has said nothing.
The beraita lists things that are forbidden by Torah law: kilayim, or mixed species planted together, orlah, fruit of a tree less than three years of age, and certain incestuous relationships, including one’s mother. One who vows that their wife is forbidden to them like any of these other things forbidden by Torah law, says the beraita, has said nothing at all. The vow simply doesn’t work. So why would the mishnah require such a vow to be dissolved? Why dissolve something that doesn’t actually exist? The Gemara offers two possibilities.
According to Abaye, the vow obviously doesn’t take effect according to Torah law. But it might still take effect according to rabbinic law. So, to cover his bases, the man needs to request that it be dissolved by rabbinic law; to take it back to the mishnah’s language: “that he will not do so lightly.” Perhaps Abaye wants to make sure that the vower takes rabbinic vows as seriously as he does biblical ones.
According to Rava, on the other hand, the vow never takes effect, either by Torah or rabbinic law. But Rava thinks not all people making vows would actually know that. A rabbinic expert would know that such a vow doesn’t work, and so would not think that they were actually prohibited from being physically intimate with their wife. But a non-expert might take such a vow seriously. So Rava suggests that the beraita and the mishnah are addressing two different audiences:
That (the beraita) is referring to Torah scholars, this (the mishnah) to one of the common folk.
If you (mistakenly) think that the vow is effective, says Rava, then you need to dissolve it. We require the effort of undoing the vow — even though we all (Torah experts that we are at this point!) know that the vow didn’t actually take effect, so that non-experts will be less likely to make such frivolous vows in the future.
Read all of Nedarim 14 on Sefaria.