Despite the fact that Plato's work has come down to our days in acceptable quantity, and certainly much more abundant than that of other even later philosophers, we cannot be entirely sure that we know all his ideas and doctrines.
Because there are several sources that mention theories of Plato that are not found in any of his works, and also the oldest references to them appear in none other than the work of his disciple Aristotle. And then there's card number seven.
For many centuries no one seemed to care much about this matter, until at the beginning of the 19th century scholars began to question whether there had really been unwritten teachings of the Athenian philosopher, and what importance they had in the Platonic corpus.
Plato always criticized the written transmission of knowledge, which he considered defective, ineffective and inferior to oral transmission. In the dialogue of him Phaedrus he lays out the reasons for not revealing his theories to everyone by writing.
Of the 18 letters that we conserve and that are attributed to Plato (5 of them considered false), number 7 is the only one on whose authenticity there is greater consensus among experts. It is the longest letter written by the philosopher, where at the same time that he narrates his stay in Syracuse under the tyrants Dionysius the Elder and Dionysius the Younger, he exposes part of his political and ethical doctrine. It is dated in the year 353 BC
But the great importance of this letter lies in the fact that it mentions that there are some teachings for the general public (the written works) and others only for the initiated (the unwritten doctrines).
And it is that Plato thought that the deepest truths could not be expressed in writing. Also that knowledge was only obtained from the combination of verbal description and sensory perception, and one had to have the knowledge before one could reach the object of knowledge.
Therefore, he states that no serious person would ever attempt to teach serious philosophical doctrines in a book or to the general public.
However, some are of the opinion that Plato revealed this knowledge in his discourse On the Good , whose content was collected and transmitted by several of his students and witnesses, such as Aristoxeno, Alexander of Aphrodisias, Speusippus and Xenocrates. Since Plato based his argument mainly on mathematics, most of them were quite confused.
In any case, the problem is that if the letter is authentic and if these unwritten doctrines have really existed, that would represent the true philosophy of Plato, this could make the works that have come down to us lose validity. But the experts do not agree, neither on one thing nor on the other.