Guide for the Perplexed, by Moses Maimonides, Friedländer tr. [1904], at sacred-texts.com
You must know that it is very injurious to begin with this branch of philosophy, viz., Metaphysics: or to explain [at first] the sense of the similes occurring in prophecies, and interpret the metaphors which are employed in historical accounts and which abound in the writings of the Prophets. On the contrary, it is necessary to initiate the young and to instruct the less intelligent according to their comprehension: those who
appear to be talented and to have capacity for the higher method of study, i.e., that based on proof and on true logical argument, should be gradually advanced towards perfection, either by tuition or by self-instruction. He, however, who begins with Metaphysics, will not only become confused in matters of religion, but will fall into complete infidelity. I compare such a person to an infant fed with wheaten bread, meat and wine; it will undoubtedly die, not because such food is naturally unfit for the human body, but because of the weakness of the child, who is unable to digest the food, and cannot derive benefit from it. The same is the case with the true principles of science. They were presented in enigmas, dad in riddles, and taught by an wise men in the most mysterious way that could be devised, not because they contain some secret evil, or are contrary to the fundamental principles of the Law (as fools think who are only philosophers in their own eyes), but because of the incapacity of man to comprehend them at the beginning of his studies: only slight allusions have been made to them to serve for the guidance of those who are capable of understanding them. These sciences were, therefore, called Mysteries (sodoth), and Secrets of the Law (sitre torah), as we shall explain.
This also is the reason why "the Torah speaks the language of man," as we have explained, for it is the object of the Torah to serve as a guide for the instruction of the young, of women, and of the common people; and as all of them are incapable to comprehend the true sense of the words, tradition was considered sufficient to convey all truths which were to be established; and as regards ideals, only such remarks were made as would lead towards a knowledge of their existence, though not to a comprehension of their true essence. When a man attains to perfection, and arrives at a knowledge of the "Secrets of the Law," either through the assistance of a teacher or by self-instruction, being led by the understanding of one part to the study of the other, he will belong to those who faithfully believe in the true principles, either because of conclusive proof, where proof is possible, or by forcible arguments, where argument is admissible; he will have a true notion of those things which he previously received in similes and metaphors, and he will fully understand their sense. We have frequently mentioned in this treatise the principle of our Sages "not to discuss the Ma‘aseh Mercabah 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" (mebin midda‘ato), as our Sages termed it.
I will now proceed to explain the reasons why we should not instruct the multitude in pure metaphysics, or begin with describing to them the true essence of things, or with showing them that a thing must be as it is, and cannot be otherwise. This will form the subject of the next chapter; and I proceed to say