Shevuot 21

There are two types of oaths that are really four: On my oath I will eat; on my oath I will not eat. On my oath I ate; on my oath I did not eat.

The opening mishnah of the third chapter of Tractate Shevuot returns us, at long last, to the subject of the tractate. The first category is future-focused, including declarations about actions that one commits to taking (“I will eat …”) and from which one commits to refrain (“I will not eat …”). The second category includes oaths about the past — what one did or did not do.

On yesterday’s daf, Rav Dimi grounded these two primary categories of oaths (future, past) in Torah verses:

When Rav Dimi came from the land of Israel, he reported that Rabbi Yohanan says: If one takes an oath, saying “I will eat” or “I will not eat,” and does not fulfill it, it is a false oath and its prohibition is derived from here: You shall not take an oath by My God’s name falsely. (Leviticus 19:12)

If one (falsely) takes an oath saying “I ate” or “I did not eat” it is an oath taken in vain and its prohibition is derived from here: You shall not take the name of the Lord your God in vain. (Exodus 20:7)

If one makes an oath about the future, it has the potential to be a truthful one — that is, until one violates it. The future act is what transforms the oath into a false one, a violation of Leviticus 19:12. By contrast, an oath taken about the past is either true or false from the moment it is uttered. If the latter, the very act of making the oath is a violation of the prohibition on taking God’s name in vain in Exodus 20:7. But in truth, Exodus 20:7 and Leviticus 19:12, though they have slightly different formulations, seem very similar overall, so it’s difficult to know exactly why each was ascribed to its particular category (past/future).

On today’s daf, the Gemara presents an alternate tradition for the biblical sources, also carried to Babylonia from the land of Israel and also taught in the name of Rabbi Yohanan:

When Ravin came from the land of Israel, he reported that Rabbi Yirmeya says that Rabbi Abbahu says that Rabbi Yohanan says: If one takes an oath, saying: “I ate,” or, “I did not eat,” it is a false oath if it is not true and its prohibition is from: And you shall not take an oath by My name falsely. (Leviticus 19:12)

If one takes an oath, saying: “I will eat,” or, “I will not eat,” and breaks his oath, he violates the prohibition: When a person vows a vow to the Lord, or takes an oath to bind their soul with a bond, they shall not break their word. (Numbers 30:3).

Ravin agrees with Rav Dimi that Rabbi Yohanan utilized Leviticus 19:12, but he reports that Rabbi Yohanan thinks that this verse refers to oaths that are focused on the past. Concerning oaths about the future, he says that Rabbi Yohanan grounds that prohibition in a third verse, Numbers 30:3, which prohibits breaking one’s word. This verse is, in fact, a slightly better match for an oath that has not yet been violated because it speaks to keeping one’s word.

Now there are three verses in play. Since Ravin’s explanation does not account for what Exodus 20:7 comes to teach us, the Gemara fills in:

And which oath is an oath taken in vain (as per Exodus 20:7)? It is when one takes an oath to deny that which is known to people to be true.

The opening mishnah suggests that oaths are, essentially, about an individual’s behavior: I ate, or not, I will eat, or not. In searching for a use of the third verse about not taking a false oath, the Gemara creates another category entirely: Oaths that deny widely-held truths. While Gemara quickly moves on, we’re left to ponder the implications of this new category.

It seems, from this statement in the Gemara, that one is forbidden from taking an oath to affirm a belief that is known to be untrue — like swearing the grass is blue or that it is summer when it is actually winter. This has the potential to get messy. If a contemporary flat-earther were to take an oath to convince others of their views, they would potentially be in violation — but so would a person who took an oath that the earth is spherical in a time when the idea the earth was flat was widely accepted. 

I find myself wishing the rabbis had taken this opportunity to engage in a philosophical conversation about the nature of truth — it would be fun to eavesdrop on that conversation. It may well have been an interesting one; especially in the absence of modern notions that truth can be relative. But the Gemara is usually more interested in law than philosophy, and that is no less true today. However, we might make a guess how that conversation would have gone if we refer to this beautiful and somewhat opaque teaching from Shabbat 55a:

​​Rabbi Hanina said: The seal of the Holy One, Blessed be He, is truth.

Read all of Shevuot 21 on Sefaria.

This piece originally appeared in a My Jewish Learning Daf Yomi email newsletter sent on May 22, 2025. If you are interested in receiving the newsletter, sign up here.