Rabban Yoĥanan ben-Zakkai had five students, namely: Rabbi Eli'ezer ben-Hyrkanos, Rabbi Yehoshu'a ben-Ĥananyah, Rabbi Yosé ha-Kohen, Rabbi Shim'on ben-Netan'el, And Rabbi El'azar ben-Arakh. He would enumerate their virtues [thus]: Rabbi Eli'ezer ben-Hyrkanos is a cemented cistern that never loses a drop. Rabbi Yehoshu'a ben-Ĥananyah - happy she who gave him birth! Rabbi Yosé ha-Kohen is pious. Rabbi Shim'on ben-Netan'el is sin-fearing. Rabbi El'azar ben-Arakh is an ever renewing source. He would say that if all the sages of Israel were on one side of the scales and Eli'ezer ben-Hyrkanos on the other he would outweigh them all. [However,] Abba Sha'ul quotes him as saying that if all the sages of Israel were on one side of the scales - including Rabbi Eli'ezer ben-Hyrkanos - and Rabbi El'azar ben-Arakh were on the other he would outweigh them all.
It seems reasonably clear from what has preceded that it was his great acumen in expounding ma'aseh merkavah that endeared Rabbi El'azar ben-Arakh to his teacher, Rabban Yoĥanan ben-Zakkai. These esoteric studies were all the vogue at the time. Indeed, throughout Jewish history as far as we can tell during times of acute stress and uncertainty scholars took to occupying themselves with esoteric and occult matters. Apparently, these disciplines involved - among other things - deep trance-like meditation. Such meditation was not without its dangers. Well known is a passage from the Tosefta [Ĥagigah 2:2] -
Four [sages] entered the orchard. Ben-Azzai, ben-Zoma, Another and Rabbi Akiva. One peeped and died; one peeped and was hurt; one peeped and tore up the roots; and one went up safely and came down safely.
The term orchard here is a code word for these esoteric studies. (Much later on it was understood as representing four aspects of such study, but it seems more likely that in our present context it was influenced by the fact that the newly reconstituted Sanhedrin in Yavneh held its conclaves in a vineyard.) Rabbi Shim'on ben-Azzai apparently was so affected by what he saw in his meditative state that he died. Rabbi Shim'on ben-Zoma, according to this passage, lost his reason ('was hurt'). 'Another' is a reference to Rabbi Elisha ben-Avuyah. (Why he was so called will become clear when we reach the twentieth mishnah of Chapter 4.) This sage was so affected by his esoteric studies that he left Judaism altogether and became an apostate.
From a midrashic passage that has survived it seems clear that the evaluation of his students (as given in our mishnah) by Rabban Yoĥanan ben-Zakkai was made before the outbreak of the great Jewish war in 66 CE. During their 'pacification' of Judah (a typically Roman euphemism for 'brutal conquest') the Romans had set aside several towns and villages that were not destroyed but used as resettlement centers for 'friendly enemies'. Yavneh was one such place, Lod was another and Emmaus was a third. There were others. As we know, after his escape from beleaguered Jerusalem Rabban Yoĥanan ben-Zakkai settled (or was settled) in Yavneh. Most of his students and colleagues joined him there and embarked on the long process of reconstruction. Apparently, Rabbi El'azar ben-Arakh did not join his teacher in Yavneh. A midrash [Kohelet Rabba 7:15] tell us that most of the students of Rabban Yoĥanan ben-Zakkai went to Yavneh but that Rabbi El'azar ben-Arakh was persuaded by his wife to go to Emmaus.
Emmaus was a kind of spa town where people went to 'take the waters' and enjoy the invigorating climate. (The town was not too far from the modern monastery at Latrun.) It was a town, it seems, in which life was easy and morals were loose. At any rate, Rabbi El'azar ben-Arakh continued there his meditative delvings into esoterica - something very dangerous to do on one's own. In Emmaus Rabbi El'azar ben-Arakh, the erstwhile star pupil of Rabban Yoĥanan ben-Zakkai, lost all his learning. According to one passage [Shabbat 147b] when some of his former colleagues finally came to find him they found that he was even unable to read Hebrew correctly! It does not seem too far-fetched to assume that this sage had lost his reason - wholly or partly.
Apparently, this was not unusual. Even Rabban Yoĥanan ben-Zakkai himself on his death-bed was hallucinating. According to the Talmud of Eretz-Israel [Sotah 47a] as he lay a-dying he instructed the bystanders to "bring a chair for Hezekiah, King of Judah".
In Avot 131 we had occasion to quote a mishnah from Ĥagigah 2:1. The mishnah reads as follows:
We do not expound 'forbidden sexual liaisons' among three people, nor 'creation' among two; and we do not expound 'the chariot' even to one person unless he is wise and already has his own perceptions of this topic.
Judith May writes:
The Mishnah you quote from Ĥagigah 2:1 is tantalizing: what does it have to say about the teaching of science and sex education?
I am not sure that it would be appropriate to draw conclusions from this mishnah concerning modern methods of education. However, the usual explanation given is that the biblical passages concerning illicit sexual relations - particularly Leviticus 18 - should not be expounded to three students at a time. The fear is that when the teacher is explaining something to one student the other two (or more) might indulge in lewd comments among themselves: if there are less than three students present that is not possible. Concerning expounding 'creation', in his commentary on that mishnah Rambam says that the reason is because
The masses are not able to comprehend these matters [easily] so they should learn them one-on-one... because [when many learn together] the unlearned among them will become bored and perhaps his faith will be impaired because he will think that [what is being taught] contradicts the [biblical] truth, whereas it [ma'aseh bereshit] is true and correct.