“You need to know things the others don't know. It's what no one knows about you that allows you to know yourself.”
Source: Point Omega
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Don DeLillo 101
American novelist, playwright and essayist 1936Related quotes
Source: This is All: The Pillow Book of Cordelia Kenn

“Reputation is what other people know about you. Honor is what you know about yourself.”
Source: Vorkosigan Saga, A Civil Campaign (1999)

as quoted by [Steven Chu and Charles H. Townes, Biographical Memoirs V.83, National Academies Press, 2003, http://www.nap.edu/catalog.php?record_id=10830, 0-309-08699-X, 201]

“There is something about yourself that you don't know.”

Other

“Don't you hate it when somebody knows you better than you know yourself?”
Homecoming saga, Earthborn (1995)

Variant translations
If you know others and know yourself, you will not be imperiled in a hundred battles; if you do not know others but know yourself, you win one and lose one; if you do not know others and do not know yourself, you will be imperiled in every single battle.
Know your enemy and know yourself, find naught in fear for 100 battles. Know yourself but not your enemy, find level of loss and victory. Know not thy enemy nor yourself, wallow in defeat every time.
Literal translation: Know [the] other, know [the] self, hundred battles without danger; not knowing [the] other but know [the] self, one win one loss; not knowing [the] other, not knowing [the] self, every battle must [be] lost.
Source: The Art of War, Chapter III · Strategic Attack