Ma'aseh Merkavahמעשה מרכבה

"The Work of the Chariot" — the older, more dangerous curriculum: Ezekiel 1 as a manual of ascent. "Not before one, unless he is wise and understands of his own knowledge." Four entered the Pardes; one came out whole. The root system of all Hekhalot literature.

The restriction quoted above is Mishnah Chagigah 2:1, and the four are Chagigah 14b — see text-chagigah. Both were paraphrased here for a long time without a citation.