Oath to Fulfill a Mitzvah

The Ran and Rosh (Nedarim 8a) argue whether an oath to fulfill a mitzvah is binding in the sense that one would be in violation of “desecrating his word” for not keeping his promise. Ran understands that an oath on a mitzvah is not binding for the purpose of being liable for a korban, but is binding, and if he transgresses the mitzvah, he has violated the prohibition against desecrating his word. The Rosh seems to understand that it is not binding at all. This is also the opinion of the Ramban brought by Reb Akiva Eiger.

Reb Avi Lebowitz points out that based on this understanding, they also argue as to what the novelty of Rav Gidal’s teaching is. The Ran understands that the oath is binding and therefore obviously not an oath taken in vain, so the novelty is that one is encouraged to make these types of oaths (even those who generally refrain from taking oaths), for it will inspire him to fulfill the mitzvah. But, according to the Rosh that the oath is not really binding, the novelty is simply that by making such an “oath,” it is not an automatic violation of an oath taken in vain, since it at least accomplishes a function of encouraging the person to fulfill the mitzvah.