“If ignorant both of your enemy and yourself, you are certain to be in peril.” 
― Sun Tzu, The Art of War

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“Anger may in time change to gladness; vexation may be succeeded by content.
But a kingdom that has once been destroyed can never come again into being; nor can the dead ever be brought back to life.” 
― Sun Tzu, The Art of War

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“Rouse him, and learn the principle of his activity or inactivity. Force him to reveal himself, so as to find out his vulnerable spots.” 
― Sun Tzu, The Art of War