This is something Proclus wrote in his Parmenides commentary that I rarely go more than 2-3 days without thinking about.
Proclus is commenting on that part of the Parmenides (at 128d) where Zeno describes how awkward it was to go viral with a philosophical thing he wrote as a young man — someone stole a copy of it and distributed it. (It’s so relatable to the Internet era, isn’t it? When I was reading the dialogue itself way back when, I was like yikes.) Anyway.
718, trans. Morrow & Dillon:
Writings of a genuinely profound and theoretical character ought not to be communicated except with the greatest caution and considered judgment, lest we inadvertently expose to the slovenly hearing and neglect of the public the inexpressible thoughts of god-like souls. The human mind cannot receive all the contents of Intellect, for there are some things known to Intellect but inconceivable by us. Nor do we think it proper to put in speech all that we think of, for there are many matters that we keep secret and unexpressed, preferring to guard them in the enclosures of our minds. Nor do we put in writing all that we express in speech; we want to keep some things in our memories unwritten, or deposit them in the imaginations and thoughts of friends, not in lifeless things. Nor do we publish indiscriminately to all the world everything that we commit to writing, but only to those who are worthy of sharing them, indulging with discrimination our eagerness to make our treasures common property with others.
(Sooner or later, I’m going to move my long live-tweet reading thread into WordPress for posterity with some additional comments I have in a Google Drive document.)