Let us begin with a summary of the sugya in Temurah [16, based on dafyomi.co.il] and we will then start asking questions - or better "dropping BOMBS" [all of these questions were asked by the Rishonim and Achronim and our analysis is based on the Be'er Miriam of HaGaon HaGadol Meod R' Dovid Mann ztz"l [Melachim 5/2]. The Maharatz Chiyus has a entire sefer called Torah Haniviim which deals with these and similar issues. Also there is a Rav from Chicago named Rabbi Sender who has written sefarim - easy to read with copious sources - that focus on these topics]: 

Don't be superficial:-)!

I would like to note that before we begin, a cursory glance at the sources we quoted do not turn up many significant problems. Daf yomi shiurim cover these sugyos in minutes. However, thought and focus reveal serious problems whose solutions create a state that can be described as ליהודים היתה אורה ושמחה וששון ויקר. And ולכל בני ישראל היה אור במושבותיהם!

Gemara in Temurah - A Navi may not reinstitute forgotten laws based on his nevuah

Rav Yehudah said - Three thousand Halachos were forgotten during the mourning over Moshe's death.

1. They asked Yehoshua to ask Hashem. He refused, for "Lo va'Shamayim Hi." They asked Shmuel (400 years later) to ask Hashem. He answered "Eleh ha'Mitzvos" שאין נביא רשאי לחדש דבר - a Navi may not teach a new (mid'Oraisa) law through prophecy [even though the law had been previously known - this is important!].

R. Yitzchak Nafcha: One of the forgotten Halachos was a Chatas whose owner died;

1. They asked Pinchas to ask Hashem. He refused, for "Lo va'Shamayim Hi." They asked Elazar to ask Hashem. He answered "Eleh ha'Mitzvos" - a Navi may not be Mechadesh Halachos.

Rav Yehudah: Before Moshe departed to Gan Eden, he asked Yehoshua to ask any doubts he had.

1. Yehoshua: (I have no doubts, for) I never left you, like you wrote in the Torah "u'Mesharaso Yehoshua... Lo Yamish mi'Toch ha'Ohel"!

2. Immediately, Yehoshua's strength waned. He forgot 300 Halachos and had 700 doubts. Bnei Yisrael wanted to kill him.

3. Hashem: I may not tell the Halachos to you. Go distract Bnei Yisrael by beginning the war - "Acharei Mos Moshe... va'Yomer Hash-m... "

Braisa: During the mourning over Moshe's death, they forgot 1,700 Kal va'Chomerim, Gezeiros Shavos and Dikdukei Sofrim (Halachos that were later quantified, e.g. 13 laws of the Nevelah of a bird of a Tahor species).

R. Avahu: Osni'el ben Kenaz restored all of them through his Pilpul - "va'Yilkedah Osni'el ben Kenaz Achi Kalev va'Yiten Lo Es Achsah Bito l'Ishah". 

End of sugya

Gemara in Megillah - A Navi MAY reinstitute previously known laws based on his Nevuah

In the gemara in Megilla the gemara says that the Neviim decreed the אותיות סופיות of מנצפ"ך. The gemara asks from the pasuk אלה המצות that a Navi is not permitted to say anything new [שאין נביא רשאי לחדש דבר]. Answers the gemara that שכחום וחזרו ויסדום - it was forgotten and the Neviim reinstituted it. 

We see from the gemara in Megilla that the pasuk of אלה המצות does not prevent the Neviim from reinstituting forgotten halachos. 

Now we are in trouble because we saw in the gemara in Temurah that based on the pasuk אלה המצות Neviim are NOT allowed to reinstitute halachos. 

Question - Arava was reinstituted based on nevuah 

The gemara in Succah [44] teaches that the custom to take the Arava in the Beis Hamikdash on Succos was from the Neviim. The gemara asks that it was actually halacha li-moshe mi-sinai. The gemara answers that it was originally halacha li-moshe mi-sinai but was forgotten and reinstituted by the Neviim al pi hadibbur - prophetically [Rashi].

How does that fit with the gemara in Temurah that Neviim may not reinstitute forgotten halachos based on nevuah because אין נביא רשאי לחדש דבר?

Two sources for the prohibition of a Navi to innovate

The gemara in Temurah offers two sources for the איסור of אין נביא רשאי דבר. One learned from the mouth of Yehoshua - לא בשמים היא. The other learned from Shmuel - אלה המצוות. 

Why the two disparate sources?

This begs for Brisk!!!

In fact, the Brisker Rov answers that לא בשמים היא teaches that one may not clarify which halachos were already given. 

אלה המצוות teaches something completely different, namely, that a Navi may not innovate new halachos based on his nevuah.

The Rambam integrates the two sources

The Brisker Rov himself points out that his theory is highly questionable because the Rambam writes [Yesodei Hatorah 9/1] "לא  "בשמים היא הא למדת שאין נביא רשאי לחדש דבר מעתה - From לא בשמים היא we derive that אין נביא רשאי לחדש דבר which is derived from ואלה המצוות. So we see that the Rambam actually fuses the two sources into one limmud and thus it is clear that according to him they are not teaching us two different laws as the Brisker Rov suggested. So why then do we need two sources to teach us the same thing? 

Why does the Rambam omit the pasuk of אלה המצות?

It is also puzzling that the Rambam omitted mention of the pasuk אלה המצות [he just alluded to it, as the כסף משנה notes]. Why does he ignore the gemaros in Temurah, Megilla, Shabbos and Yoma which quote the pasuk??

Does claiming that one received a halacha through prophecy turn one into a false prophet?

The Rambam writes [Yesodei Hatorah 9/1]: 

הא למדת שאין נביא רשאי לחדש דבר מעתה לפיכך אם יעמוד איש בין מן האומות בין מישראל ויעשה אות ומופת ויאמר שה' שלחו להוסיף מצוה או לגרוע מצוה או לפרש במצוה מן המצות פירוש שלא שמענו ממשה או שאמר שאותן המצות שנצטוו בהן ישראל אינן לעולם ולדורי דורות אלא מצות לפי זמן היו הרי זה נביא שקר שהרי בא להכחיש נבואתו של משה ומיתתו בחנק על שהזיד לדבר בשם ה' אשר לא צוהו שהוא ברוך שמו צוה למשה שהמצוה הזאת לנו ולבנינו עד עולם ולא איש אל ויכזב:

This teaches that a prophet can no longer add a new precept [to the Torah].

Therefore, if a person will arise, whether Jew or gentile, and perform a sign or wonder and say that God sent him to:

a) add a mitzvah,

b) withdraw a mitzvah

c) explain a mitzvah in a manner which differs from the tradition received from Moses, or

d) if he says that the mitzvot commanded to the Jews are not forever, but rather were given for a limited time,

he is a false prophet. He comes to deny the prophecy of Moses and should be executed by strangulation, because he dared to make statements in God's name which God never made.

God, blessed be His name, commanded Moses that this commandment is for us and our children forever, and, God is not man that He speak falsely. 

We learn from this Rambam that a false prophet is one who adds or subtracts or gives a false interpretation to the mitzvos. 

However, later on [halacha 4] the Rambam says that he is a false prophet if he claims that Hashem taught him a halacha through prophecy: 

או שאמר בדין מדיני תורה שה' צוה לו שהדין כך הוא והלכה כדברי פלוני הרי זה נביא השקר ויחנק

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.

He again writes explicitly in his preface to Seder Zraim that the very claim that Hashem taught him a law, renders him a false prophet [and NOT because he was contradicting the Torah]. How does that dovetail with what the Rambam writes earlier where this is omitted and one is only considered a false prophet if he contradicts the Torah?  

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 [14] and Bava Metzia [49] 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 earlier]

או שאמר בדין מדיני תורה שה' צוה לו שהדין כך הוא והלכה כדברי פלוני הרי זה נביא השקר ויחנק

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.   

Can Eliyahu decide whether tefillin are kosher if written on the flesh of a fish?

The gemara in Shabbos [108] asks if one may write tefillin on the skin of a fish. The gemara answers that Eliyahu Hanavi will came and tell us. The gemara asks what this means? Explains Rashi that heter and issur are not dependent on the opinion of Eliyahu because of the rule לא בשמים היא. 

The Maharatz Chayus asks according to Rashi's understanding how the gemara often says תיקו which is an acronym for ת"שבי י"תרץ ק"ושיות ו"בעיות. So the gemara itself says in many instances that it is Eliyahu who will decide this question.  

The answer is that Chazal have the status of the Beis Din Hagadol [see R' Elchonon Kuntres Divrei Sofrim #4] and they determined that in instances where we have no הכרעה  [clear halachic decision], we may follow the dictates of Shomayim. However this does not always apply and sometimes Chazal did not say that we are bound by the decisions rendered from Shomayim. Therefore, regarding the issue of writing tefillin on the flesh of a fish where we have no basis to assume that we must follow the Heavenly dictates, the gemara asked how Eliyahu can rule this question.

So now we have pshat in the gemara in Shabbos. B"H!!!

When R' Evyatar met Eliyahu why didn't he ask him to resolve all of the times it says Teiku?

We see from the gemara in Gittin [6b] that Eliyahu used to appear to Rav Evyatar. The Birkei Yosef asks in the name of Rav Betzalel Ashkenazi why Rav Evyatar didn't take advantage of these oportunuites to ask Eliyahu to resolve all of the s'תיקו in Shas. He answered that there were two different Eliyahus... The gemara in Menachos [45b] says about a certain puzzling pasuk that Eliyahu will expound it in the future. How can Eliyahu rule if לא בשמים היא? He answers in the name of the Rashba that Eliyahu has the power to explain and interpret dinim. But what about the gemara in Shabbos [the fish skin] where the din is not allowed to be interpreted by Eliyahu?? Answered the Birkei Yosef that Eliyahu may decide the halacha based on his intellect and logic and not on any Divine inspiration [see also Ri"z Halevi on Parshas Pinchas who explains that Eliyahu was part of the chain of transmission of the Torah that the Rambam listed and he therefore has the right to decide halachic questions]. But if that is correct then how come the gemara in Shabbos [according to Rashi] wonders how Eliyahu is ruling on the fish skin question? Let us say that he is ruling based on logic and not prophecy?? See what he answers.....

Chazal can tell us to follow Eliyahu

But we can explain as follows: Rav Betzalel Ashkenazi was asking why R' Evyatar didn't ask Eliyahu to resolve all of the Teiku's in Shas. As we explained, Chazal [as Beis Din Hagadol] gave him this right and ability. The Rashba [Menachos 45] is referring to the gemara that says מקרא זה עתיד אליהו לדורשו  - so Chazal gave him their stamp of approval and he had the right to interpret that specific din. However the gemara about the fish skins is talking about a case where Eliyahu was NOT given the right to decide, so the gemara correctly asks why it matters what he rules when we know that לא בשמים היא. 

R' Chiya chose to follow Eliyahu

In Chullin [36a] R' Chiya is in doubt whether we say ישנה לשחיטה מתחילה ועד סוף - if the shchita process is considered continual from beginning to end [or only starts at the end]. The gemara says that the question is in limbo "לעולם" which Rashi understands to mean that Eliyahu will decide. The Maharatz Chayus asks on Rashi from the gemara about the fish skins where we see that Eliyahu may not decide halachic questions. So how can Eliyahu decide this question??? 

The answer is as we said earlier that the psak and הכרעה of Eliyahu is not binding but if it does not conflict with any of our rules of psak [such as rov] we have the right to follow him. R' Chiya chose to follow Eliyahu's psak in the future. The gemara in Shabbos is talking about Mar the son of Ravina who DIDN'T choose to follow Eliayhu's psak. If so, why is it binding?! That is the gemara's question.