exception proves the rule, the

Idioms  

1. An instance that does not obey a rule shows that the rule exists. For example, John's much shorter than average but excels at basketball—the exception proves the rule. This seemingly paradoxical phrase is the converse of the older idea that every rule has an exception. [Mid-1600s]

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Example Sentences

Prov′er.—The exception proves the rule, the exception tests the rule, proving its general truth.

From Chambers's Twentieth Century Dictionary (part 3 of 4: N-R) by Various