Avot172

16:
It is reasonable to assume that no Tanna was able to master the whole corpus of Tannaitic material that had amassed. Just think of all the material in the six orders of the Mishnah, all the material in the Halakhic midrashim (Mekhilta, Sifra and Sifré), all the material in the Tosefta and much of the material in the Aggadic midrashim. It is beyond human capability for one person to recall that amount of learning. There were 'professional memory men' (also designated by the title Tanna). These were people of prodigious memory who undertook to remember, word for word, whole segments of the oral tradition. No self-respecting Bet Midrash would have been without at least one Tanna such as this. (The sages preferred their Tanna ['memory man'] to be someone who was not also a sage, so that the Tanna's ignorance of halakhic argumentation would help preserve the accuracy of what he remembered.)

17:
The only way in which all this vast amount of material could be remembered is by constant repetition. The repetition had to be unwavering in its accuracy, and no doubt students and sages would assess the accuracy of their recollection by comparison with each other. We get an insight into what it meant to be a sage's student [talmid ĥakham] from what is almost an aside in the Gemara [Sanhedrin 99a-b]:

18:
It is now a little easier to understand the severity of the teaching of Rabbi Shim'on ben-Yoĥai in our present mishnah. If a student or sage is using the opportunity afforded by walking from one place to another to review some part or other of their learning – 'singing' along the road – the accuracy of their repetition could be ruined by extraneous comments. Almost every sage would have agreed with him that such comments were most detrimental to the learning process; but surely it was only the dour and uncompromising character of Rabbi Shim'on himself that lead him to threaten such dire consequences: "Anyone who is walking down a road repeating his learning in his head and says,'What a beautiful tree!' is considered by scripture as having put his life in jeopardy."

19:
Surely, it is the very severity of this judgement that prompted the editor of our tractate to juxtapose to the teaching of Rabbi Shim'on the teaching of his no less great younger contemporary, Rabbi Me'ir (quoted by his student Rabbi Dostai). First of all Rabbi Me'ir supplies the biblical source hinted at by Rabbi Shim'on [Deuteronomy 4:9]: