According to professor Arthur M. Melzer, in his 2014 book Philosophy Between the Lines: The Lost History of Esoteric Writing, until the 19th century philosophers were well-known to have provided both exoteric (public) and esoteric (private) versions of their philosophy.
Aristotle was known for this dual communication in his lectures, and Proclus said of Plato that since it was unbecoming to speak of the most divine of dogmas before the multitude,
Plato himself asserting that all these are ridiculous to the many, but in an admirable manner are esteemed by the wise. Thus also, the Pythagoreans said, that of discourses, some are mystical, but others adapted to be delivered openly. With the Peripatetics likewise, some are esoteric, and others exoteric; and Parmenides himself, wrote some things conformable to truth, but others to opinion; and Zeno calls some assertions true, but others adapted to the necessary purposes of life.
Professor Melzer explains that classical and medieval writers understood that if they spoke openly of their beliefs they would suffer consequences. Some, like Pythagoras and Socrates, wrote nothing. Others, like Plato and Aristotle, wrote but may have conveyed the esoteric only through oral teachings to selected students.
But there were also those who wrote exoterically with the esoteric writings hidden “between the lines” through hints and insinuation, and perhaps through coded language. They wrote in a multi-level way.
Saint Augustine in the fourth century A.D. in one of his letters believed that the pure stream of philosophy should be available for only the few and kept away from the common herd. “I think that that art of concealing the truth is a useful invention.”
An Arabic philosopher in the tenth century, Al-farabi, wrote a commentary on Plato’s Laws in which he states:
Wise Plato did not reveal all his knowledge to all people. He used symbols and riddles, wrote in veils and made the text a challenge, so that knowledge would be protected from the undeserving who would change it, and from those who, not knowing its value, would use it poorly. He was correct to do this. Once he became renowned for this practice, he would occasionally state a topic more openly and literally; but some readers still assume he is being symbolic or obscure, intending something different from the literal. This idea remains as one of his greatest secrets. Only those trained in that art of two-level and secret meanings will understand Plato.
Although my purpose here is not to pretend to know such two-level and secret meanings, I’ve included some Neoplatonic writings and commentaries, primarily through Thomas Taylor, so that the reader may get a glimpse of what may be hidden.
The Two Traditions
There are essentially two major approaches to the inquiry into Truth: one tradition, primarily Eastern, of learning directly from a master or guru, through direct experience and revelation; and another tradition, primarily Western European, of intellectual philosophical inquiry through argument and reasoning.
Perhaps ironically, Plato’s dialogues embody both. Socrates himself did not believe in writing down his teachings. He believed that writing down philosophy resulted in a loss of memory, a kind of forgetting that undermined a moral culture by letting a person become lazy. For when something is written, you do not have to remember it, or exercise your own mind and imagination to properly own it yourself.
He also faulted writing because you cannot question it, like you can a person, and expect an answer. He explains why in the Phaedrus:
I cannot help feeling, Phaedrus, that writing is unfortunately like painting; for the creations of the painter have the attitude of life, and yet if you ask them a question they preserve a solemn silence. And the same may be said of speeches. You would imagine that they had intelligence, but if you want to know anything and put a question to one of them, the speaker always gives one unvarying answer. And when they have been once written down they are tumbled about anywhere among those who may or may not understand them, and know not to whom they should reply, to whom not: and, if they are maltreated or abused, they have no parent to protect them; and they cannot protect or defend themselves.
However, we cannot help but be grateful that Plato wrote what Socrates did not.
The idea that Socrates sought to impose his ideas of Truth, Justice, and Virtue on others is contradicted by him in the Theaetetus, among others, where he attributes to his inner divine guide a restriction against his bring forth his views:
Well, my art of midwifery is in most respects like theirs; but differs, in that…the triumph of my art is in thoroughly examining whether the thought which the mind of the young man brings forth is a false idol or a noble and true birth. And like the midwives, I am barren, and the reproach which is often made against me, that I ask questions of others and have not the wit to answer them myself, is very just—the reason is, that the god compels me to be a midwife, but does not allow me to bring forth. And therefore I am not myself at all wise, nor have I anything to show which is the invention or birth of my own soul, but those who converse with me profit.
So in a very direct sense, Socrates is in engaging in a Master/Student pursuit where the Student discovers the way himself with the help of the Master. Plato, on the other hand, wrote and schooled and set the course for the Western tradition of philosophy. As Alfred North Whitehead said:
The safest general characterization of the European philosophical tradition is that it consists of a series of footnotes to Plato.
from the Editor’s Introduction: The Best Complete Plato