Like every teacher, I suppose, I've had more than one talk with a student who said: "I understand it all just fine in my mind, or when you say it or I read the textbook, but when I try to write it down, I just can't seem to say what I mean." One semester I had heard this line often enough that I posted on my door this rejoinder from Montaigne's essay "On the Education of Children, written around 1579-1580.
"I hear some making excuses for not being able to express themselves, and pretending to have their heads full of many fine things, but to be unable to bring them out for lack of eloquence. That is all bluff. Do you know what I think these things are? They are shadows that come to them of some shapeless conceptions, which they cannot untangle and clear up within, and consequently cannot set forth without: they do not understand themselves yet. And just watch them stammer on the point of giving birth; you will conclude that they are laboring not for delivery, but for conception, and that they are only trying to lick into shape this unfinished matter."The quotation is from "The Complete Works of Montaigne: Essays, Journal, Letters," as translated by Donald M. Frame (Hamish Hamilton; London, p. 125).