Maimonides
Maimonides' 12th Century work, Guide for the Perplexed is in part intended as an explanation of the passages Ma'aseh Bereshit and Ma'aseh Merkabah. In the third volume, Maimonides commences the exposition of the mystical passage of the mystic doctrines found in the merkavah passages, while justifying this "crossing of the line" from hints to direct instruction. Maimonides explains basic mystical concepts via the Biblical terms referring to Spheres, elements and Intelligences. In these chapters, however, there is still very little in terms of direct explanation.
"We have frequently mentioned in this treatise the principle of our Sages " not to discuss the Maaseh Mercabhah even in the presence of one pupil, except he be wise and intelligent; and then only the headings of the chapters are to be given to him." We must, therefore, begin with teaching these subjects according to the capacity of the pupil, and on two conditions, first, that he be wise, i.e., that he should have successfully gone through the preliminary studies, and secondly that he be intelligent, talented, clear-headed, and of quick perception, that is, " have a mind of his own", as our Sages termed it." Guide for the Perplexed,Read more about this topic: Merkabah