There are also "Plato's unwritten doctrines" that supposedly "could not be explained in writing in a way that would be accessible to general readers, [so] their

There are also "Plato's unwritten doctrines" that supposedly "could not be explained in writing in a way that would be accessible to general readers, [so] their dissemination would lead to misunderstandings. Plato therefore supposedly limited himself to teaching the unwritten doctrines to his more advanced students in the Academy. The surviving evidence for the content of the unwritten doctrines is thought to derive from this oral teaching." https://en.wikipedia.org/wiki/Plato%27s_unwritten_doctrines https://en.wikipedia.org/wiki/Plato%27s_unwritten_doctrines