The Mishna's Authenticity
Reader: How can we be sure, that the Mishna brings us the "Torah she beal peh" that was delivered in Sinai? Maybe it was just what Rebi Yehuda Hanasi observed in his generation?
Mesora: If that is the case, why didn't our Talmudic Sages suggest this? They realized that unanimous acceptance is verified proof that the previous generation attested to this. This is our Mesora – tradition – going back to Moses.
Reader: You are arguing that: 1) There was unanimous acceptance. 2) Unanimous acceptance by an ideological group proves their ideology.
Mesora: Yes, mass conspiracy to the event of Sinai where we learned the Oral and Written Law cannot be fabricated. Man cannot create a mass conspiracy.
Reader: However, both of these statements are false. a) there was no unanimity, and there were myriad opposition groups. The Sadducees existed prior to the Talmudic and Mishnaic period, from the beginning of the 2nd temple era.
Mesora: The Talmud proves unanimity as stated above. Sadducees confirm the truth of the law given at Sinai. Why didn't they simply state Sinai never occurred? They couldn't, because it did, and their attempt to denounce Oral Law meant they perceived the Oral law. One cannot denounce what does not exist.
But you must ask, who determines the truth of a system: the original recipients, or those who come later? This is the very same argument against Christians attempting to redefine various verses in our Torah. They oppose Rabbinical interpretations, which preceded them.
To determine the truth of nay system, we look to those who received it initially.