In continuing our discussion of the differences between a neder and a shevua, the Mishna teaches that nedarim can take effect on mitzvot while shevuot cannot. The example given by the Gemara is that a person who forbids on himself a sukka, a lulav or tefillin will be obligated to fulfill his neder even if he can no longer perform these commandments. If he takes an oath that he will not perform these mitzvot, however, he is still obligated to do them, since ein nishba’im la’avor al ha-mitzvot – a person cannot take an oath to abrogate a mitzva.
The explanation offered by Abaye as to why a person can take a neder not to do mitzvot, even as he cannot make a shevua not to do them, fits in with the ideas that we have already learned with regard to these laws. Abayye teaches that the neder – which works – has the man saying “the pleasure derived from sitting in the sukka is forbidden to me,” while the shevua – which does not work – has him saying “I swear that I will derive no pleasure from the sukka.” Since the Torah commands every man to sit in a sukka, the oath that a man takes to refrain from doing – which aims to create a prohibition on the person – contradicts the Torah’s command. This stands in contrast with the person who creates a prohibition on the object – the sukka – which is not an object of mitzva in and of itself.
The Talmud Yerushalmi suggests that the difference stems from the foundations of nedarim in the world of kodashim. As we have learned, nedarim are ordinarily expressed in language where the person compares the object of the vow to a holy object, for example, “this meat should be to me like a sacrifice”. Just as a sacrifice is forbidden, so this meat becomes forbidden by means of the neder. In our case, since kodashim can take effect on any object – even on objects of a mitzva – similarly nedarim have the power to do so.