An Enduring Text
‘If One Vows By The Torah…’
A Mishna in the third chapter of Tractate Shevuos (27b) discusses the Torah’s prohibition to swear falsely, as stated (Exodus 20:7), “You shall not take the Name of the L-rd your G-d in vain.” The Mishna explains that if one did so willfully, in the knowledge that he was swearing falsely, he is liable to makkos (lashes). If he did so unwittingly, however, he is exempt.
A Mishna in the following chapter (Shevuos 35a) states that in the event one swore falsely [using the expression] “by heaven and earth,” he is exempt from a penalty because he did not use Hashem’s Name.
The Kesef Mishneh (Rambam, Hilchos Shevuos 12:4) understands from the latter text that this is not considered an oath at all and therefore no transgression is involved.
Our Gemara (Nedarim 14b) states that if one utters a neder (vow) while holding a Torah scroll, the vow is binding. The Ran (ad loc.) and Rambam (supra) explain that the Gemara often substitutes the term neder for shevuah (oath).
If one holds a Torah scroll while uttering an oath, the oath is valid because the person uttering the oath is effectively saying “by it,” meaning by what is written in the Torah scroll, including the Holy Names of G-d inscribed therein.
Only A Torah Scroll?
Rambam, in codifying this halacha, distinguishes between a Sefer Torah and the Books of the Prophets and Scripture. Rambam (loc. cit.) states that only swearing on a Sefer Torah is a binding oath, but if one swears on one of the Books of the Prophets it is not considered a valid oath.
Any Sacred Text
Ravad (ad loc.) disputes this distinction and proves, based on a Gemara in Tractate Shabbos, that not only a Torah scroll but the Books of the Prophets are also sacred texts because the Holy Names of G-d are inscribed therein. He thus maintains that an oath taken on [one of] the Books of the Prophets is certainly valid.
Only Eternal Texts
Tzafenas Pa’ne’ach (cited by Kuntress Yosef Da’as) links this controversy to another dispute between Rambam and Ravad in Hilchos Megilla: Rambam submits that in the era of Moshiach, all the books of Scripture will become obsolete, except for the Five Books of Moses and the Scroll of Esther. Ravad vehemently disagrees, maintaining that no book of Scripture will ever become obsolete.
Rambam is thus of the opinion that if one swears on a Book of the Prophets, the oath is not binding since it will not endure. [Even though Megillas Esther is an exception and will endure forever, it cannot be used for an oath because the Name of Hashem does not appear in it.]