Contradictions

A copy of a post from last year:

In tractate Kiddushin, Mishna states that a man cannot betroth two sisters at the same time. If one betroths a group of women (e.g., five) at the same time, and amongst them are two sisters, the sisters are not betrothed (even though the other women are). The sisters are not legally betrothed — to the point that if one of them has a child from another man, the child is not a mamzer.

Gemara asks the question: what if the man gives two sisters a gift each and says “You are betrothed to me” to both of them, at the same time? Is one of them betrothed? Should the man write them both a get?

Rava answers, quoting Rabba who said: “What cannot happen in sequence cannot happen even simultaneously”. I.e., if B cannot happen after A, it cannot happen at the same time as A. If A cannot happen after B, it cannot happen at the same time as B. When A and B happen simultaneously, they are negating each other — B is impossible, since it is as if it happened after A, and vice versa.

So, neither of the sisters is betrothed, and a get is not necessary.

* * *

In tractate Nedarim, the question of vow revocation or confirmation is discussed. If a woman makes a vow, her husband or father (depending in whose house she is living at the moment) has to uphold it in order for the vow to be valid or revoke it in order for the vow to become invalid.

What if the husband (or father) says: “I am confirming and revoking the vow at the same time”? Gemara quotes Rabbah again: “Anything that cannot happen in sequence cannot happen simultaneously either”.

Seemingly, the halacha is that since revokation cannot happen after confirmation of the vow (once it’s confirmed, it cannot be revoked) and confirmation cannot happen after revokation (once a vow is revoked, it cannot be confirmed — a new vow needs to be made), the two cancel each other, and the vow is neither confirmed nor revoked.

Rambam, however, rules in Mishnei Toireh:

If he tells her: “It is upheld for you and nullified to you at the same time”, the vow is upheld. If he tells her: “It is upheld to you today”, it is upheld forever. If he tells her: “It is nullified for you tomorrow”, it is not nullified, for he upheld it today and he cannot nullify it on the following day.

What is Rambam doing? Is he contradicting Gemara? If not, how is it possible to reconcile Rambam with the simple reading of Gemara?

Brisker Rav writes in Hilchos Nedarim: The essence of upholding a vow contradicts ability to nullify the vow. Once a vow is upheld, it is legally impossible to nullify it. The essence of revoking the vow, however, does not contradict legally the idea of upholding it; rather, it removes physical posibility of upholding it (by destroying the vow). Therefore, since upholding the law nullified revoking it, but not vice versa, in the case of simultaneous proclamation of upholding and revoking, the law is upheld.

This is different from the case of marrying two sisters. There, the idea of marrying sister A legally contradicts the idea of marrying sister B — and vice versa. Therefore, when an attempt to do both happens, the two attempts are able to legally contradict and nullify each other. In the case of vows, since only one attempt legally contradicts the other, and the second merely physically, circumstantially contradicts the first (as it happens, once a vow is revoked, it stops existing, so there is nothing to uphold), the attempt to revoke does not legally contradict the attempt to uphold, when done simultaneously — and therefore, the halacha is that the vow is upheld.

Why does Gemara bring the opinion of Rabbah then? Exactly to show this principle: things that cannot happen in succession cannot happen simultaneously either. Since revocation cannot happen after confirmation, it cannot happen at the same time either. Therefore, revocation is nullified. Stop. The principle doesn’t go the other way, as one could think from the simple reading. This, says the Brisker Rav, is what Rambam sees as the reason for bringing the opinion of Rabbah.

This sheds light on the first case as well: there, instead of one event (cancellation of both betrothals, to both sisters), two events happen — a cancellation of the betrothal to sister A (by an attempt to betroth sister B) and a cancellation of the betrothal to sister B (ditto).