Ran Elucidated - Daf 16

Distinction between a vow and an oath - The Gemora cites the Scriptural source for this: How do we know that one cannot swear to transgress a mitzvah? It is because it is written [Bamidbar 30:3]: If a man makes a vow to Hashem or makes an oath to prohibit himself, he shall not desecrate his word; according to whatever came out of his mouth, he shall do. We see from here that only “his word” (a discretionary matter) he should not desecrate. However, he may desecrate it when it involves the wishes of Heaven (where his oath contradicts a mitzvah).

The Gemora asks: Why is a vow any different? If it is because it says makes a vow to Hashem, which implies that he should not desecrate his word even when his vow involves the wishes of Heaven, why, by an oath, it is also written makes an oath to Hashem (since the words “to Hashem” is written between a vow and an oath)?

Abaye answers: In the case of a vow, one says, “The pleasure of the sukkah should be forbidden upon me.” (Since the vow is upon the object, not the person, it is effective.) However, in the case of an oath, one says, “I swear that I will not derive any pleasure from the sukkah.” (Since the oath is upon the person and the mitzvah is upon the person, they are contradicting each other and the oath cannot take effect.)

The Ran explains: It is reasonable that the word “to Hashem” should be referring to a vow, but not an oath. It is logical to assume that a neder in this manner will take effect. This is because he is not vowing directly against the Torah’s commandment. The neder takes effect and he is prohibited from deriving benefit from the sukkah because we cannot force a person to eat something that is forbidden to him. However, by an oath, he is saying that he will not sit in the sukkah. This is a direct contradiction to the Torah’s commandment and he doesn’t have the power to free himself from the Torah’s commandment to sit in a sukkah.