Prov. Something that does not follow a rule shows that the rule exists. (Often used facetiously, to justify some rule you have proposed but which someone else has listed exceptions. From a Latin phrase meaning that an exception tests a rule.) Ellen: Men are always rude. Jane: But Alan's always polite. And Larry and Ted are polite, too. Ellen: They're just the exceptions that prove the rule.Bill: All the shows on TV are aimed at people with low intelligence. Alan: What about that news program you like to watch? Bill: The exception proves the rule.