The Seventh Letter of Dionysius

Letter suggests plato didn't think it was possible to state philosophical truth in a book, hence his preference for dialogues over didactic treatises. Diogenes says that plato invented the use of dialogue form in writing philosophy. Perhaps he was always working through his ideas and never reached a doctrine that satisfied him. Then again, maybe plato was perfectly confident of his insights, but worried about the vulnerability of words, especially written words.