In ancient times skillful warriors first made themselves invincible, and then watched for vulnerability in their opponents.
Invincibility is in oneself, vulnerability is in the opponent.
Therefore skillful warriors are able to be invincible, but they cannot cause opponents to be vulnerable. That is why it is said that victory can be discerned but not manufactured. Invincibility is a matter of defense, vulnerability is a matter of attack. Defense is for times of unsufficiency, attack is for times of surplus. Those skilled in defense hide in the deepest depths of the earth, those skilled in attack maneuver in the highest heights of the sky.