לזכות אלחנן בן הענא מרים שיזכה ללמוד וללמד מתוך נחת והרחבת הדעת וכל טוב גשמי ורוחני
When may we follow a Bas Kol?
It would appear that the basis of the law of לא בשמים היא is not that a halachic decision cannot be rendered from Above but that the decision is not binding even though it came down from Above. Halacha must be decided based on the laws of hermeneutics [מדות שהתורה נדרשת בהן] and logic, so a decision arrived at through other means is not binding.
That is the meaning of what Tosfos say in Yevamos  and Bava Metzia  that despite the fact that in the machlokes between R' Eliezer and the Rabbis we don't pay heed to the Bas Kol because of the principle of לא בשמים היא, nevertheless in the machlokes between Beis Shammai and Beis Hillel we do rule based on a Bas Kol. Because where the Rabbis constitute the majority we ignore the Bas Kol unlike the machlokes between Beis Shammai and Beis Hillel where Beis Hillel was the majority but we were in doubt that maybe we should still follow Beis Shammai because they were sharper. In such a case we follow the Bas Kol [and rule like Beis Hillel].
This leaves us unsatisfied. After all is said and done, we are in doubt as to whether we should follow Beis Shammai [who are sharper] or Beis Hillel [who are the majority] and we decide to follow Beis Hillel based on a Bas Kol. This flies in the face of our rule that לא בשמים היא!
The Kesef Mishna asks on the Rambam who holds that we NEVER follow a Bas Kol, how is he going to deal with Tosfos' question that we see that we do follow the Bas Kol in the machlokes between Beis Shammai and Beis Hillel. The Rambam cannot answer as Tosfos does that in certain instances we follow the Bas Kol because he doesn't hold that way. The Kesef Mishna remains with his question unresolved. But as we said, Tosfos themselves require an explanation. [See R' Elchonon, Kuntres Divrei Sofrim 5-9,12,13].
There is another puzzling issue that requires clarification. If it is true that לא בשמים היא and therefore we don't pay heed to Bas Kols, then why on earth [or in Heaven] does the Bas Kol call out. Don't they hold in Shomayim that לא בשמים היא?
The Shittah Mekubetzes [Bava Metzia 49] asks this question in the name of Rav Nissim Gaon and offers two answers, see there. We may answer that in Shomayim they agree that their Bas Kol is not binding. So the Bas Kol makes herself heard and we may follow it to resolve a doubt if it does not contradict our laws of psak. Therefore, when the Bas Kol comes out and says that the halacha follows Beis Hillel we may follow it because it is in line with our laws of psak. [Only according to R' Yehoshua we completely disregard a Bas Kol - see Tosfos Yevamos 14 ד"ה רבי but we don't pasken like R' Yehoshua.]
This is explicit in the Rambam in his peirush hamishnayos:
שהנביא כשיסבור סברא ויסבור כמו כן מי שאינו נביא סברא ויאמר הנביא כי הקב"ה אמר אלי כי סברתי אמת, לא תשמע אליו רק אלף נביאים כולם כאליהו ואלישע יהיו סוברין סברא אחת ואלף חכמים וחכם סוברין הפך הסברא ההיא אחרי רבים להטות וההלכה כדברי האלף חכמים וחכם וכו' ע"כ.
We see that there is a הכרעה, a halachic decision that may be rendered by a Navi, however when there is a majority against this decision then it is not accepted as binding and we follow the majority.
The Rambam writes in the fourth halacha [quoted in the previous post]
or states with regard to one of the Torah's laws that God commanded him to render such and such a judgment, or that such and such is the law regarding a particular instance and the decision follows a certain opinion, he is a false prophet and should be [executed by] strangulation.
The explanation is that the false prophet claims that his Nevuah is BINDING and that is why he is considered a false prophet and executed. But nevertheless, as we said, a prophecy received by the Navi is also a valid opinion and may be followed if there are no other factors involved that would prevent us from following it [such as a majority].
So the question of the Kesef Mishna that we mentioned [how is the Rambam going to explain the fact that we followed the Bas Kol that told us to pasken like Beis Hillel] is answered. We MAY follow a Bas Kol [even according to the Rambam] because it is a valid opinion no less than the opinion of a chochom. Since we were in doubt as to whether we follow Beis Hillel [who has the majority on their side] or Beis Shammai [who were sharper], we listened to the Bas Kol that told us to follow Beis Hillel [just as the Tosfos we quoted answers]. And it is not correct to say [as did the Kesef Mishna] that the Rambam holds that we may NEVER follow a Bas Kol [or any prophetic communication] and that it should be completely disregarded. That is only true when the claim is that the prophecy OBLIGATES us to follow its bidding. But if it is just presented as an opinion, then we may follow it in the absence of other mitigating factors.
THIS IS GREAT - SO MUCH MORE TO SAY.
ועוד חזון למועד!