Aristotle tells us that Plato had an unwritten doctrine or unwritten dogmas (agrapha dogmata). Commenting on Plato’s Timaeus, Aristotle recorded:
“It is true, indeed, that the account he gives there of the participant is different from what he says in his so-called unwritten teaching.”
- Aristotle, Physics 209b
This secret teaching conforms to something said by Plato in his Seventh Letter:
“Every serious man in dealing with really serious subjects carefully avoids writing” (344c).
So what is this secret “unwritten doctrine” of Plato? I can’t write it down. Come to class and find out…
Taylor Marshall is the Adjunct Instructor in Philosophy at the University of Dallas. 
So the symbol for this is a circle, a straight line, and two smaller circles on the other side of the line, right?
My take on that is the line is the One in the invisible realm, and dualism in the visible. The dualism being the ghost in-a-machine type of doctrine, as when Plato has Socrates speak of soul and body in terms of body and clothes.