Why is "Moses was the greatest of prophets" a principle of faith?

Question

The 6th and 7th of the Rambam's 13 principles of faith (ikkarim) are:

6. God revealed Himself through the prophets
7. Moses was the greatest of prophets

Why the 7th as a separate principle? The Rambam himself gives four reasons:

-God did not speak to the other prophets face to face.
-God spoke to them in their sleep or when they were in a trance, not in broad daylight.
-When God would speak to them they would lose their faculties because of the intense experience [Dan. 10:8ff, 10:16].
-They could not communicate with God whenever they wanted to.

None of this applied to Moses.

That's all well and good, but I still don't see how it adds to what Jews believe. With the same logic, you could spell out what the Messiah will do, what the World to Come will imply, etc. Why is that 7th principle there, instead of subsuming it in the 6th?

Answer 1

TL;DR While Moshe Rabbeinu was a prophet, his prophecy was fundamentally different than all other prophets.

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R' Herschel Schachter in a TorahWeb article titled "Torah and Nevuah" quotes a dynamite question from the Minchas Chinuch:

The Rambam (Yesodei HaTorah 9:2) understands the mishna in Sanhedrin 89a as saying that "one who violates the instructions of a Navi" deserves misa beyedei shomayim (death by heavenly court).

The Minchas Chinuch (#516) is bothered with a most obvious problem: wasn't Moshe Rabbeinu a Navi? It should therefore follow that anyone who violates any Biblical law given by Moshe Rabbeinu ought to deserve this punishment of misa beyedei shomayim!

R' Schachter notes how various suggestions over the years have been offered to this question, yet the generally accepted approach today was offered by his rebbe, Rav Y.D. Soloveitchik.

Rav Soloveitchik understands that "one who violates the instructions of a Navi" ONLY applies to one who violates a hora'as sha'ah (temporary ruling)... [however] the 613 mitzvos- a binding force for all generations, and which were only given by Moshe Rabbeinu- are NOT included in this category. They have their own system which has its own hierarchy of punishments. "Over al divrei novi" applies only to one who violates a "dvar nevuah".
Some of Moshe Rabbeinu's instructions were "divrei nevuah," while most were elevated to the level of "divrei Torah" and "mitzvah" because of their binding force for all generations.

R' Schachter (cont.) says that the Rambam distinguishes between Moshe Rabbeinu and other prophets in this manner. The only prophet who was ever given mitzvos (with a binding force for all future generations) was Moshe Rabbeinu. Moshe's prophecy was the only prophecy that was on the level of Torah.

Answer 2

R. Joseph Albo discusses this in Sefer HaIkarim 1:3:

Our first question concerning these principles which Maimonides laid down is, why precisely those thirteen? If it is because of the meaning of the term 'ikkar' (fundamental principle), we can see very well the reason for including such dogmas as the existence of God, prophecy, revelation, omniscience, providence in reward and punishment, because they are essential principles of a divine law, which cannot be conceived without any one of them. We may also allow without difficulty the inclusion of Moses as the greatest prophet and the perpetuity of the law, because they are special principles, essential to the law of Moses which can not be conceived without them. For if we assume that there may be a prophet greater than Moses, it follows that the law of Moses may be abolished, for the words of a greater prophet should be believed more than those of a prophet who is inferior in degree, as we shall see later. (Husik translation Vol. I p. 56-57, my emphasis)

Answer 3

First, many Rambam's opponents question the idea of the 13 principles being the "true" principles of Judaism (and the practical application for deciding who's called a heretic, for example).

Second, In my understanding, based on R' Margaliot's book, in those principles, Rambam is trying to "build" reasonable and consistent proof of the Torah's validity as opposed to other religions.

As #6 (prophecy) is not exclusive to the Jewish people, other nations may claim that their prophets receive other messages or texts. A firm belief that Moses was different from them all will, therefore, prove the superiority of our Torah to their texts.