Our Gemara on Amud Beis discusses various scenarios of making an oath when there already is an existing Torah obligation. (Known as mushba v’omed mehar Sinai, which we discussed on Psychology of the Daf, Shavuous 21.) The Gemara raises the possibility that since the prohibition for many consumables is only from a kzayis and more, perhaps an oath to not eat a certain forbidden food should be valid. The oath will find its footing on the prohibiting of even less than a kzayis, that an oath can add on. For example, a Nazir who is forbidden to eat grape products that are a kzayis could make an oath not to eat even less than a kzayis of grapes, and it should have clearance to be valid because of this additional prohibition beyond his vow of nazirhood.

Tosafos (“aheteira”) suggests that this general idea is applicable even according to Rabbi Yochanan’s opinion that less than a kzayis of consumables is also forbidden by Torah law (it just takes a kzayis for it to be a formal, legally punishable transgression). Tosafos says that the idea of the pre-existing oath does not apply to prohibitions or commandments in the Torah that are not specific legal prohibitions.

There is a similar idea mentioned by the Ran in Nedarim (8a “Ha Kamashma”) that if the commandment or prohibition isn’t explicitly written in the Torah, it does not fall under the rubric of mushba v’omed, a preexisting oath from the obligation at Sinai.

What is the logic behind Tosafos and the Ran? Perhaps we might say that the perception of obligation is part of the process. The oath only feels meaningful if one is adding a prohibition. So, if there is a minor prohibition but not part of the typical or conscious awareness of the covenantal requirements of the Torah, then the oath adds a degree of prohibition. This idea might even be what the Gemara in Nedarim (8a) means when it says that an oath to fulfill a Torah obligation is permitted because it motivates him. We might say the very fact that it somehow adds motivation shows that, at least subjectively, there is some perception of additional prohibition or command, and so, in effect, there still is space for the oath to find footing.

I heard in Rav Sruly Bornstein’s Daf Yomi Shiur an interesting related Chovos Halevavos and question from Rav Shlomo Zalman Auerbach ZT’L. Chovos Halevavos (Shaar Avodas Hashem 3) asserts that while one might naturally assume there are three types of behaviors: Forbidden, Commanded, and Permitted (that is, the morally neutral acts), this is not true. Chovos Halevavos says everything is either a mitzvah or a sin because all activities should be weighed and measured. Even if one eats or sleeps, if it is in service of God, such as to be healthy to do His will, it is a mitzvah. If it is purely for personal gratification, it is sinful.

While this Chovos Halevavos raises a high bar, it also leads Rav Auerbach to ask, “How then can one make an oath on any activity? It also should fall under the principle of mushba v’omed mehar Sinai as a preexisting oath?”

If one follows Tosafos or the Ran, this question is answerable because certainly the Chovos Halevavos’ demand for constant moral discernment for every action is not explicit in the Torah. I am wondering, based on the Akeidas Yitschok we discussed in blog post Psychology of the Daf Shavuous 21, how would he answer Rav Shlomo Zalman’s question? After all, Akeidas Yitschok held the oath was metaphorical and really represented the recognition of the inevitable bond and responsibility of the Torah and the covenant. If so, should that not include even non-explicit but morally apparent demands for fealty to God’s will, such as what the Chovos Halevavos states? I think if we understand the idea that it is about perceived obligation, as I suggested above, this gives us a way to posit how the Akeidas Yitschok would explain it. Only moral obligations that would be felt universally and self-evidently would fit under the principle of mushba v’omed because these would block a sense of space for adding on a new prohibition via an oath. But if it were subtle obligations that are not consciously held, the new oath would be perceived as adding a level of prohibition and thus find a foothold.