“You know the Devil is your enemy but you do not deal with him as such.”
Ibrahim ibn Adham (718–776) ascetic Sufi saint
Source: The Sayings and Teachings of the Great Mystics of Islam (2004), p. 28
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
“You know the Devil is your enemy but you do not deal with him as such.”
Ibrahim ibn Adham (718–776) ascetic Sufi saint
Source: The Sayings and Teachings of the Great Mystics of Islam (2004), p. 28
“To know your Enemy, you must become your Enemy.”
Sun Tzu (-543–-495 BC) ancient Chinese military general, strategist and philosopher from the Zhou Dynasty
This is sometimes attributed to Sun Tzu in combination with the above quote, as well as alone, but it too has not been sourced to any published translation of The Art of War, though it is similar in concept to his famous statement in Ch. 3 : "It is said that if you know your enemies and know yourself, you will not be imperiled in a hundred battles..."
Misattributed
“If you are your own worst enemy, don't do yourself any favors.”
Tom Heehler American author
The Well-Spoken Thesaurus (2011)
Miyamoto Musashi (1584–1645) Japanese martial artist, writer, artist
Go Rin No Sho (1645), The Fire Book
Muhammad al-Taqi (811–835) ninth of the Twelve Imams of Twelver Shi'ism
[Baqir Sharīf al-Qurashi, The life of Imam Muhammad al-Jawad, Wonderful Maxims and Arts, 2005]
“You don’t always win your battles, but it’s good to know you fought”
Lauren Bacall (1924–2014) American actress, model