Much of today’s daf (Nedarim 78a) draws on the various interpretations ascribed to the expression זֶה הַדָּבָר – ‘this is the word’ (Bemidbar 30:2) – which is how the Torah introduces the laws of vows and oaths. And among other things, our daf quotes a Beraita noting how this same expression of זֶה הַדָּבָר is also used with reference to the laws forbidding sacrifices outside of the temple (שחוטי חוץ, see Vayikra 17:2) – thereby establishing what is known as a ‘Gezera Shava’ (גזירה שוה) between these two rules. Yet what isn’t clear to the reader is what this comes to teach us.
To answer this question, we need to understand the significance of the statement זֶה הַדָּבָר as used elsewhere in the Torah, and as the Meshech Chochmah (and others) point out, the phrase זֶה הַדָּבָר almost always refers to a temporary rule (ie. one referring to a particular moment or period of time, but with little to no application beyond that moment or period of time). For example, the phrase זֶה הַדָּבָר is used in Shemot 35:4 when speaking of the contributions to the Mishkan, and in Vayikra 9:6 when describing the process of the dedication of the Mishkan.
However, there are some instances when the laws introduced by the phrase זֶה הַדָּבָר are not temporary but are, instead, permanent. This is the purpose of the Gezera Shava referenced in our Gemara (and originally discussed in Bava Batra 120b) to teach us that in contrast to the other examples in the Torah where זֶה הַדָּבָר speaks of a temporary rule, there is a unifying principle binding the laws of vows with the laws forbidding sacrificing outside of the temple (שחוטי חוץ) – namely that notwithstanding the usage of the phrase זֶה הַדָּבָר in both these passages, these are laws which apply forever.
But this itself is curious. Why would the Torah employ a phrase which is ordinarily used about particular moments or periods of time when discussing a law which, as deduced from the Gezera Shava, is a ‘forever’ rule (לדורות)?
My belief is that this duality points to a deeper concept relating to vows themselves – which are often said in the heat of a moment but which can often have long lasting effects. Simply put, the dual meaning of זֶה הַדָּבָר points to these dual aspects of a vow. Of course, as we explain in today’s daf, a vow can be revoked or annulled. Still, as we are also taught in today’s daf, vows shouldn’t be taken lightly. And I believe that all of this is contained in the two meanings of זֶה הַדָּבָר – words that speak of the timely and the timeless; of things that are temporary, and of things that last forever.