The exception proves the rule
(If there were no rule, no exception would be needed)

Cicero's defense of L. Cornelius Balbo (56 B.C.) is the earliest known citation of this logic and is sometimes cited as the origin of the phrase. Balbo was accused of having been illegally granted Roman citizenship. The prosecutor pointed out that treaties with some non-Roman peoples prohibited granting them citizenship and suggested this should be inferred in Balbo's case. Cicero replied "If the exception makes such an action unlawful, where there is no exception the action must necessarily be lawful." (Quod si exceptio facit ne liceat, ubi <non sit exceptum, ibi> necesse est licere.)

Those who read Latin can read the whole speech.