The laws of vows and the rabbinic cautions against making them teach the holiness and power of the spoken word.
The following article is reprinted with permission from Kolel: The Adult Centre for Liberal Jewish Learning.
Parashat Mattot begins with a detailed presentation of the laws pertaining to vows and oaths. Next, Moses is instructed to "take revenge" against the Midianites, and there is a long report on Israel's terrible battle against Midian. In the aftermath of the war, Moses reminds the soldiers about tumah--the laws of ritual impurity--and deals with the division of booty between the soldiers, community, and the Mishkan (Tabernacle). Next, Moses is approached by the tribes of Reuven and Gad, asking to be apportioned some land on the east side of the Jordan River. At first, Moses is annoyed by this request, but he then relents as long as they agree to continue to fight with the rest of Israel to conquer the land of Israel.
If a man makes a vow to the Eternal or takes an oath imposing an obligation on himself, he shall not break his pledge; he must carry out all that has crossed his lips (Numbers 30:3).
This seems pretty straightforward: If you make a promise, you must keep it. However, the text uses two very different terms here to make its point. Neder, translated as "vow," is generally used to represent a promise to do something ("I vow to give $1000 to tzedakah"). Shevu'ah, on the other hand, is generally translated as "oath," implying a promise to abstain from doing something ("I swear to stop smoking"). In each case, as soon as it is uttered, the promise is considered binding. A man must carry through what ever he states. And the text does refer to men here.
The passage continues to discuss what happens when a woman makes a vow or an oath. In that case, an unmarried woman's father or a married woman's husband can annul her vow if they object to it as soon as they hear about it. If they do not object, then it is binding as stated, just as with a man.
The Torah considers oaths and vows to be serious business. As our passage stipulates, this is especially true since an oath or vow is a pledge to God. As it states in Ecclesiastes 5:4, "It is better that you should not vow, than that you should vow and fail to fulfill." The Rabbis also took oaths as a matter of great concern. An entire tractate of the Talmud, called Nedarim, is devoted to the discussion of oaths and the implications of making oaths.