The Rambam continues his Introduction by laying out the exact figures responsible for transmitting Torah teachings over the centuries.
Summary: Moses’ primary disciple (and successor) was Yehoshua (Joshua). Moses would teach Yehoshua first, who would then teach the Seventy Elders (see Exodus 24:1, Numbers 11:16) who, in turn, taught the rest of the nation. The Prophets and High Priests transmitted the teachings throughout the following generations, all the way up to the Knesset HaGedolah, the “Great Assembly” of 120 Sages, which consisted of history’s last prophets (including Zechariah, Malachi, Ezra, and Nehemiah), and the first rabbis (namely Shimon haTzadik). The rabbinic period officially began at this point. The tradition continued to be passed on until Rabbi Yehuda haNasi, “the Prince” (c. 135-217 CE), who took the monumental step of first putting the Oral Torah into writing. He did this because Roman persecutions had nearly extinguished Judaism, and Rabbi Yehuda (often referred to simply as Rebbi) felt that the Oral Tradition must be recorded for preservation, lest it be lost and forgotten. The Code of Law that Rebbi produced came to be known as the Mishnah.
Insight: The Rambam is famous for being a strict rationalist. He generally avoided speaking of mystical matters and rarely relied on Midrash. (Midrash refers to the allegorical level of Torah study.) Here, in listing the chain of transmission, the Rambam surprisingly notes Achiya the Shilonite who, according to Midrash, merited to live for hundreds of years. He participated at the Exodus from Egypt, and lived all the way up to the times of Eliyahu (Elijah), as we read in I Kings 11 and 14. The Rambam specifically notes Achiya as a link between Moses and Eliyahu – a time span of over 500 years. We would think that the rationalist Rambam would not rely on such a Midrash! Indeed, he does make it clear that there were, of course, many other links in the chain between Moses and Eliyahu, including Pinchas (who was undoubtedly at the Exodus, as we read in the Torah), Eli the High Priest, Shmuel (Samuel), and King David. We must also not forget that there was an entire era of Shoftim, “Judges”, like the righteous prophetess Deborah, and the great hero Shimshon (Samson) who were certainly links in the chain of Torah transmission as well. Altogether, we see a solid, unbroken chain of Torah transmission going all the way back to Mt. Sinai!