Believing in the Prophets

Rambam in Yesodei HaTorah Perek 8:
ו נִמְצֵאתָ אוֹמֵר, שֶׁכָּל נָבִיא שֶׁיַּעֲמֹד אַחַר מֹשֶׁה רַבֵּנוּ, אֵין אָנוּ מַאֲמִינִין בּוֹ מִפְּנֵי הָאוֹת לְבַדּוֹ, כְּדֵי שֶׁנֹּאמַר אִם יַעֲשֶׂה אוֹת נִשְׁמַע לוֹ לְכָל מַה שֶׁיֹּאמַר; אֵלָא מִפְּנֵי הַמִּצְוָה שֶׁצִּוָּנוּ מֹשֶׁה בַּתּוֹרָה, וְאָמַר אִם נָתַן אוֹת, "אֵלָיו, תִּשְׁמָעוּן" (דברים יח,טו): כְּמוֹ שֶׁצִּוָּנוּ לַחְתֹּךְ הַדָּבָר עַל פִּי שְׁנֵי עֵדִים, וְאַף עַל פִּי שְׁאֵין אָנוּ יוֹדְעִין אִם אֱמֶת הֵעִידוּ אִם שֶׁקֶר; כָּךְ מִצְוָה לִשְׁמֹעַ מִזֶּה הַנָּבִיא, אִם הָאוֹת אֱמֶת אוֹ בְּכִשּׁוּף וְלָאט

The halacha is that if a person comes along and says he is a navi, we don't believe him immediately. Rather, we must test the person. He has to make a prediction that comes true that establishes the he is in fact a navi. Once a navi establishes that he is a true navi by this method, we believe that he is a navi and we must listen to his words. There are more details to these halachos which can be found in the 7th through 10th chapters of Hilchos Yesodei HaTorah in the Rambam.

The Rambam there establishes that our belief as a nation in Moshe Rabbeinu's nevuah did not come about through the miracles that Moshe performed. For, as the Rambam says, anyone can fool someone with magic. Rather, the nation believed in Moshe Rabbeinu because they actually witnessed Hashem speaking to Moshe on Har Sinai. Miracles alone according to the Rambam cannot establish full emunah.

If so, why do we believe in a navi just because he performs a "miraculous" prediction. Perhaps he is fooling us with magic?

To this the Rambam answers here - yes perhaps it is so. Still, we believe the navi because that is the halacha. Just like the halacha is we believe two eidim even though they may be lying. Nevertheless, this is the standard the halacha established to believing eidim. So too, the standard the halacha has for believing a navi is based on performance of a miracle, despite the fact that he may be fooling us.

For this reason, a navi can never come along and argue with Toras Moshe Rabbeinu. The idea being that the only reason we believe the navi is because Toras Moshe says we must. Otherwise, the miracle alone wouldn't establish independent belief in the navi. So, obviously, if we are only believing the navi because it says to believe him in the torah, so certainly his word cannot overturn the word of the Torah.

[Note: the halacha is different if the navi is only overturning the halacha b'horaas shaah like Eliyahu at Har HaCarmel. See the Rambam inside for details.]