A man always is to be himself the judge of how much of his mind he will show to other men; even to those he would have work along with him. There are impertinent inquiries made: your rule is to leave the inquirer uninformed on that matter; not, if you can help it, misinformed; but precisely as dark as he was!

Thomas Carlyle (1795-1881) Scottish essayist and historian

Lecture (1840-05-22), “The Hero as King,” Home House, Portman Square, London
    (Source)

The lecture notes were collected by Carlyle into On Heroes, Hero-Worship, & the Heroic in History, Lecture 6 (1841).