The Art of War (Chinese: 孫子兵法; pinyin: Sūnzǐ bīngfǎ; literally: “Master Sun’s Rules of Warfare”) is an ancient Chinese military treatise dating from the 5th century BC. Attributed to the ancient Chinese military strategist Sun Tzu — “Master Sun”, the text is composed of 13 chapters, each of which is devoted to one aspect of warfare. It is commonly thought of as a definitive work on military strategy and tactics. It was placed at the head of China’s Seven Military Classics upon the collection’s creation in 1080 by Emperor Shenzong of Song, and has long been the most influential strategy text in East Asia. It has had an influence on Eastern and Western military thinking, business tactics, legal strategy and beyond. (Wikipedia)
Hence the saying: If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. If you know neither the enemy nor yourself, you will succumb in every battle.