It is derived from the medieval Latin legal principle: exceptio probat regulam in casibus non exceptis (the exception tests the rule in the cases not excepted).
- "Entry is free on Sundays."
- This implies that entry is not free on the other six days of the week.
- "Parking prohibited on Sundays."
- This implies that parking is not prohibited on the other six days of the week.
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.