Zhī jǐ zhī bǐ, bǎi zhàn bú dài.
Translation: If you know yourself and know the enemy, you can face 100 battles and not be in danger.
Meaning: To succeed, you need to know both your own and your opponents’ strong points and weak points well.
This saying comes from The Art of War (Sūnzǐ bīng fǎ 孙子兵法). This book about military strategies is attributed to Sunzi 孙子, who lived during the late Spring and Autumn