Why Aharon, Itamar and Elazar were not part of the 70 elders?

Question

Rambam Sanhedrin 2:2 says that the Kohanim are the first to sit in the Sanhedrin:

"וּמִצְוָה לִהְיוֹת בְּסַנְהֶדְרִין גְּדוֹלָה כֹּהֲנִים וּלְוִיִּים שֶׁנֶּאֱמַר (דברים יז ט) "וּבָאתָ אֶל הַכֹּהֲנִים הַלְוִיִּם"."

BT Eruvin 54b presents the method of Moses' Torah study:

תנו רבנן: כיצד סדר משנה? משה למד מפי הגבורה. נכנס אהרן ושנה לו משה פירקו נסתלק אהרן וישב לשמאל משה נכנסו בניו ושנה להן משה פירקן ... נכנסו זקנים ושנה להן משה פירקן נסתלקו זקנים נכנסו כל העם ושנה להן משה פירקן...

What was the order of teaching the Oral Law? Moses learned directly from the mouth of the Almighty. Aaron entered ...and Moses taught him his lesson ... Aaron’s sons entered, and Moses taught them their lesson ... The elders entered and Moses taught them their lesson ... the entire nation entered and Moses taught them their lesson.

According to this Gemmorah, neither Aharon nor his two remaining sons were part of the 70 elders. Pinchas is not mentioned at all. This contradicts the Rambam's conclusion. BTW Rambam himself brings this Gemmorah in his Introduction to Mishnah.

How come?

PS: I think it is needless to say that the Sanhedrin is "thought to be" the 70 elders.

Answer 1

The inital group of 70 were explicitly commanded to be brought from the 12 tribes (6 from each tribe). As explained by Rashi in Beha'aloscha 11:2, 70 of the 72 from the twelve tribes were chosen by lot and Eldad and Medad remained behind to avoid embarassing whoever was not chosen.

Rav Hirsch explains on Beha'aloscha 11:23 that this is designed to show that everything that is coming to pass will be by natural means. That is why Moshe appointed the 70 elders (the first Sanhedrin) before the events described later. It is implied that Moshe Rabbeinu was commanded by Hashem to appoint them and that they were named by the goral of Hashem (since the Torah does not say everything Hashem told Moshe that he told the people in 11:24)..

This also gives us the reason why the election had to precede the event, which after all had the purpose of satisfying a temporary requirement of the people.

Since the members of the first Sanhedrin were chosen by the explicit command of Hashem, the principle cited by the Rambam did not apply.

Since this did not have anything to do with inheriting the land, then I would say that the tribe of Levi was probably one of the twelve and the tribe of Yosef was treated as one. However, this is my own conjecture and I do not have a source for this.

Rashi

They were among those written: Among those chosen for the Sanhedrin. All of them were written down, mentioned specifically by name, but [the number was chosen] by lot, because the number [of elders] for twelve tribes came to six for each tribe, except for two tribes who would receive only five each. Moses said, “No tribe will listen to me to deduct one elder from its tribe.” What did he do? He took seventy-two slips and wrote on seventy [of them, the word] ‘elder’ and two of them he left blank. He then chose six men from each tribe, totaling seventy-two. He said to them, “Draw your slips from the urn. Whoever picked [one inscribed with] ‘elder’ was [already] sanctified. Whoever picked a blank slip, he said to him,”The Omnipresent does not want you." - [Sifrei Beha’alothecha 1:42:21, Sanh. 17a]