Binding Deeds To Speech

ועיקר הנדר הוא כח הדיבור דכ' ויפח באפיו נשמת חיים ויהי כו' לנפש חי' לרוח ממללא לכן אמרו הנודר כנודר בחיי המלך שהוא חיות אלקות וכן כל הדברים בק"ש ותפלה וברכות שהם כח הנשמה לקשר בזה מעשה הגוף אל הנשמה ככל היוצא מפיו יעשה שימשיך המעשה אחר הדיבור זה לאסור איסר על נפשו

The essence of a neder (vow) lies in the power of speech, for it is written, "And He breathed into his nostrils the breath of life, and man became a living soul" (Bereishis 2:7) — which Onkelos renders as "a speaking spirit"; therefore the Sages said that one who takes a vow is like one who vows by the life of the King, for speech is the life-force of Divinity. And so it is with all matters of Krias Shema, tefillah, and berachos, which are the power of the neshamah, through which one binds the deeds of the body to the neshamah: "according to all that comes out of his mouth he shall do" (Bamidbar 30:3) — that one should draw his actions to follow his speech, and this is the meaning of "to bind a binding upon his soul."

The Sfas Emes teaches that a neder draws its strength from the faculty of speech, which is the Divine life-force of the neshamah breathed into man, and the whole purpose of a vow — like Shema, tefillah, and berachos — is to attach the actions of the body to the neshamah by making one's deeds follow his words.

ואם אין המעשה נמשך אחר הדיבור נעשה פגם גם בהדיבור כמ"ש לא יחל דברו:

But if the deed is not drawn along to follow the speech, a flaw is created even in the speech itself, as it is written, "he shall not profane his word" (Bamidbar 30:3).

When a person fails to carry out in action what he has declared in speech, the defect does not remain in the deed alone but reaches back and damages the very power of speech, which is why the Torah warns "he shall not profane his word."

Summary: The Sfas Emes explains that the power of a neder flows from the faculty of speech, which is the Divine breath of life that Hashem breathed into man — the "speaking spirit" that is the life-force of the neshamah. For this reason Chazal compared one who vows to one who swears by the life of the King, since speech itself is Godliness within man. The avodah of nedarim, like that of Krias Shema, tefillah, and berachos, is to bind the deeds of the body to the neshamah by ensuring that one's actions follow his words — "according to all that comes out of his mouth he shall do." When the deed does not follow the word, however, the flaw reaches back and damages the very kedushah of speech, which is why the Torah cautions "he shall not profane his word."