From Rabbi Aryeh Kaplan, “Meditation and Kabbalah” (Samuel Weiser or Jason Aronson editions), p. 26-27. Rav Hai Gaon (939-1038) was the last exilarch, i.e., head of the Babylonian academy at Pumbeditha in the period following the redaction of the Talmud. His rulings in Jewish law were definitive in his day; in addition, he was reputed by medieval kabbalists to have been a master of the mystical tradition.
Cited in “HaKotev” on Ein Yaakov, Chagigah 12b, based on a responsum in Teshuvot ha-Ge'onim.
Rav Hai Gaon on Divine Visions
Many sages maintain that one who possesses all the necessary qualifications has methods through which he can gaze at the Merkava (Divine Chariot) and peek into the chambers on high. One must first fast for a certain number of days. He then places his head between his knees, and whispers into the ground many songs and praises known from tradition.
From his innermost being and its chambers he will then perceive the Seven Chambers. In his vision, it will be as if he is entering one chamber after another, gazing at what is in each one.
There are two tractates in which this is taught. These are called the Greater Hekhalot and the Lesser Hekhalot, as is well known.
It is with regard to such an experience that the Talmud teaches, “Four entered the Orchard” (Chagigah 12b). The chambers are likened to an orchard and are given this name. The four who entered the Merkava and passed through the Chambers are likened to people entering an orchard....
It is taught that Ben Azzai gazed and died. This is because it was his time to leave the world. It is also taught that Ben Zoma gazed and was stricken. This means that he became insane because of the confounding visions that his mind could not tolerate. He was like the “stricken ones,” regarding which the 91st Psalm was written (Shabbat 15b).
When the Talmud states that the “Other” [Rabbi Elisha ben Avuyah] “cut his plantings,” it is again using the allegory of the orchard. Since one of the four did irreparable damage, he is likened to one who enters an orchard and cuts down its trees. The Other assumed that there are two Authorities, very much like the Magii, who believe in Ormuzd and Ahriman, as well as independent domains of good and evil, like light and darkness. This is the intent of the Talmud.
Rabbi Akiba was the most perfect of them all. He gazed properly, not exceeding his limitations, and his mind was able to encompass these mighty confounding visions. God gave him power so that as long as he gazed he kept proper thoughts in his mind and maintained a proper mental state.
This was known to all the early sages and none denied it. They maintained that God would accomplish wonders and fearsome things through the saints, just as He did through the prophets. They do not deny the Talmudical accounts of miracles, such as those involving Rabbi Chanina ben Dosa and the like (Berakhot 34b).
When Mar Rav Samuel Gaon [who headed the academy between 730 and 748 c.e.] and others like him flourished, they began to read the books of the philosophers. They claimed that such visions were only seen by the Prophets, and that only the Prophets could invoke miracles. They denied all the accounts which told of miracles occurring to the saints. They say that this is not Law. The same is true of the account of Rabbi Akiba's gazing into the Chambers, and the account of Rabbi Nehuniah ben Hakana and Rabbi Ishmael. Regarding all these, they say it is not the Law.
But our opinion remains that God does wonders and miracles to His saints, and also allows them to see the Chambers.