"There is a limit to what a mind can do" expresses that a person has a clear standard of right and wrong and a moral bottom line in his mind, and has restrictions and constraints on his own behavior. The "ruler" in this sentence symbolizes the inner norms and moral constraints, reminding people to always be alert to whether their actions conform to moral standards. Only by inner fear and knowing what you should and shouldn't do can you show respect and obedience to the moral bottom line in your external behavior.