exception that proves the rule
- A form of argument in which the existence of a counterexample to a rule is used to demonstrate the fact that a rule exists.
- (idiomatic) The rare occurrence of a counterexample to a rule, used to underscore that the rule exists
a form of argument
- World Wide Words
- “The exception that proves the rule”, in Gary Martin, The Phrase Finder.
- The Straight Dope