Moshe spoke to the heads of the tribes of the Children of Israel, saying: "This is the thing that Hashem has commanded. If a man takes a vow to Hashem or swears an oath to establish a prohibition upon himself, he shall not desecrate his word; according to whatever comes from his mouth shall he do" (Bemidbar 30:2-3).

What many do not realize, though, is that sometimes what we say actually constitutes a vow, although we had not really intended it to be so. For example, often one is enthusiastic to express his confirmation or rejection of a proposal and repeats the word "yes" or "no" twice. The Gemara (Shavuos 36a) says that saying "yes, yes" or "no, no" constitutes making an oath. Even answering "amen" to a proposal may be considered an oath. This is especially true when performing a mitzvah is involved. Therefore, a person should train himself to always say "beli neder" which is a clear negation of his acceptance upon himself an obligation as a vow. For example, if someone asks, "Will you come with us tomorrow?" he should respond, "Yes, beli neder."

There are those who are exceptionally careful about fulfilling anything which they said, even if they really did not mean it at the time. I remember when my great mentor, Hagaon Reb Ya'akov Kaminetsky zt"l approached his ninetieth birthday. We were very surprised when he began to inquire into the logistics of purchasing tefillin according to the version of Rabbeinu Tam. Usually, this additional pair is worn by Chassidim, not by rabbis of Lithuanian origin. Seeing our surprise, Reb Ya'akov, who was known to be the epitome of honesty, explained that he was, for many years, the Rosh Yeshiva of Torah VaDa'as. Since many of the students were Chassidic, they were uncomfortable with a Rosh Yeshiva who did not don Rabbeinu Tam's tefillin. The Menahel Ruchani of the Yeshiva, Hagaon Reb Shrage Feivel Mendlowitz zt"l, suggested to Reb Ya'akov that he begin wearing them, but he refused, explaining that Lithuanian rabbis do not. Reb Shrage Feivel argued that the Chofetz Chaim did wear them, at the age of ninety, when he lived among Chassidim. Reb Ya'akov replied, "When I'll be ninety, I'll start wearing them too."

Although Reb Ya'akov's comments had only been meant in jest, as he approached his ninetieth birthday, he was concerned that they may have constituted a vow, since he had forgotten to say beli neder. Consequently, he purchased a pair and began to don them daily as he had promised.

We should always be cautious to think carefully before we speak, and, once we have spoken, to keep our word. Then we will be happy in this world and in the World-to-Come.