This quote by Pierre Corneille indicates that it is wise to prevent or leave from somebody who knows your weaknesses. When somebody knows our vulnerabilities, they can use them versus us and trigger us damage. This could be in the type of emotional adjustment, physical attacks or exploiting our weaknesses to get a benefit over us. Therefore, it is important to be careful of those who understand our weaknesses and take actions to safeguard ourselves from possibly damaging scenarios. It is better to be safe than sorry and prevent fight with those who might cause us damage.