Daf Yomi Yevamot 102a: If Eliyahu Should Come -- A Case of Absence of Evidence vs. Evidence of Absence

Is absence of evidence equal to evidence of absence? There is a statement trotted out on occasion that lo` ra`inu `aino ra`aya - "'We have not seen' is not a proof." (But it resonates better in Hebrew.) An example to consider is found in the beginning of Chullin, which lists who may and who may not slaughter lechatchila and bedieved. Women are not on the list, and presumably can shecht even lechatchila. And indeed Tosafot in discussing it brings proofs that this is the case from other gemaras. Meanwhile, Tosafot quotes Hilchot Eretz Yisrael that women should not shecht because "da'atan kalot" -- however you want to interpret it. (E.g. they are delicate such that they will faint a moment at the sight of blood and thus will invalidate the shechita, or else that they will treat the mitzvah lightly.) (It is actually Hilchot Amar Yehoshua, from Eldad haDani.)

In Shulchan Aruch, well:

The Shulchan Aruch in Yoreh Dayah says that everyone is able to shecht lichatchila, even a woman. The Rama states that it is permitted for women to shecht but it is not our minhag, and therefore it should not be done. The Shach comments on the Rama, and says the reason why it is not the minhag is because it is never seen, but that is not a reason for women not to shecht. Therefore the Shach holds that it is totally mutar for a woman to be a shochet.

This is thus an example of lo ra`inu aino ra`aya -- how can you establish a minhag that something is not done. It might just not have been common and thus not seen. Though some interpret this Rema as saying that since it is not our custom, women never had a chance to be mumchin, experts, and thus it is a problem.

Regardless, I believe the same idea can be brought to bear on our gemara in Yevamot 102a:

אמר רבה אמר רב כהנא אמר רב אם יבא אליהו ויאמר חולצין במנעל שומעין לו אין חולצין בסנדל אין שומעין לו שכבר נהגו העם בסנדל ורב יוסף אמר רב כהנא אמר רב אם יבא אליהו ויאמר אין חולצין במנעל שומעין לו אין חולצין בסנדל אין שומעין לו שכבר נהגו העם בסנדל מאי בינייהו איכא בינייהו מנעל לכתחלה

Rabbah stated in the name of R. Kahana in the name of Rab: If Elijah should come and declare that halizah may be performed with a foot-covering shoe, he would be obeyed; [were he, however, to declare that] halizah may not be performed with a sandal, he would not be obeyed, for the people have long ago adopted the practice [of performing it] with a sandal.

R. Joseph, however, reported in the name of R. Kahana in the name of Rab: If Elijah should come and declare that halizah may not be performed with a foot-covering shoe, he would be obeyed; [were he, however, to declare that] halizah may not be performed with a sandal, he would not be obeyed, for the people have long ago adopted the practice [of performing it] with a sandal.

What is the practical difference between them? — The practical difference between them is [the propriety of using] a foot-covering shoe ab initio.

Thus, these two statements from Rav are viewed by the setama digemara as contradictory, with contrasting practical differences.

And this is seen in the Rif's summary as well, in which he adopts Rabba's version:

Rif:

חלצה במנעל חליצתה כשרה
אמר רבא אמר רב כהנא אמר רב [אסי] אם יבא אליהו ויאמר חולצין במנעל לכתחילה שומעין לו אין חולצין בסנדל אין שומעין לו שכבר נהגו העם בסנדל
ומנעל לכתחילה מ"ט לא
גזירה משום מנעל המרופט אי נמי משום חצי מנעל

"If he performed chalitza on her with a {foot-covering} shoe, it is valid":
Rava {our gemara: Rabba} cited Rav Kahana who cited Rav [Assi]: If {the prophet} Eliyahu would come and tell us that we may perform chalitza with a {foot-covering} shoe -- ab initio {lechatchila} -- we listen to him. That we do not perform chalitza with a sandal, we do not listen to him, for the nation is already accustomed to use the sandal.

And the shoe, ab initio, why not? As a decree because of a flabby {or burst} shoe; alternatively because of half a shoe.

Thus, Eliyahu is coming to tell us that we may use a shoe even lechatchila. Thus, it appears that a shoe, bedieved, is allowed, but lechatchila is not allowed.

However, we can cast it in another light, where both statements attributed to Rav are correct. At issue is not lechatchila or bedieved, but rather whether, in the absence of existing practice, we can use a determination -- such as Eliyahu's coming and telling us -- to overturn precedent, or rather, lack of precedent. And further, compare that to where we actually have positive precedent.

We read a bit later in the gemara (same daf):

For it was taught: R. Jose related, 'I once went to Nesibis where I met an old man whom I asked, "Are you perchance acquainted with R. Judah b. Bathyra?" and he replied, "Yes; and he in fact always sits at my table". "Have you ever seen him arranging a halizah ceremony for a sister-in-law?" [I asked]. "I saw him arranging halizah ceremonies many a time", he replied. "With a foot-covering shoe [I asked] or with a sandal?" — "May halizah be performed", he asked me' "with a foot-covering shoe?" I replied: Were that [not] so, what could have caused R. Meir to state that halizah if performed with a foot-covering shoe is valid, while R. Jacob reported in his name that it was quite proper to perform [even] halizah ab initio with a foot-covering shoe!'

Thus, practically, we never see anyone perform chalitza with a foot-covering shoe, but only with a sandal. Thus,

Rabbah stated in the name of R. Kahana in the name of Rab: If Elijah should come and declare that halizah may be performed with a foot-covering shoe, he would be obeyed; [were he, however, to declare that] halizah may not be performed with a sandal, he would not be obeyed, for the people have long ago adopted the practice [of performing it] with a sandal.

That is, the negative precedent of not using a foot-covering shoe is not a proof nor a practice that one should not do it (either lechatchila or perhaps even bedieved), so Eliyahu can overturn it. However, using a sandal for chalitza is a positive precedent, such that Eliyahu cannot overturn that long-standing precedent.

Meanwhile, Rav Yosef reported to complementary case.

R. Joseph, however, reported in the name of R. Kahana in the name of Rab: If Elijah should come and declare that halizah may not be performed with a foot-covering shoe, he would be obeyed; [were he, however, to declare that] halizah may not be performed with a sandal, he would not be obeyed, for the people have long ago adopted the practice [of performing it] with a sandal.

Given that there is no precedent in actual practice, can Eliyahu come and declare that one should not use a foot-covering shoe (whether lechatchila or perhaps even bedieved)? Recall that there is a Mishna that says it is valid, though it unclear whether that means lechatchila or bedieved. Can he overturn that and say that in fact you cannot do it and that, say, even bedieved it does not work? (Or if the Mishna intended lechatchila, can he say that one should not do it lechatchila, though bedieved it would work?) Since there is no contradictory practice of using a foot-covering shoe, Eliyahu has the ability to make such a statement and be hearkened to. But for a sandal, since there is precedent in existing practice, he would not be listened to.

Thus, in sum, given a situation in which something is not done, we do not take that as proof, and Eliyahu can come and overturn the halacha either way. However, where we have positive precedent, he cannot come and overturn it.