“Exception that proves the rule” – where does the saying come from?

“Exception that proves the rule” (Exceptio probat regulam) – these were the words that come up during the defense of Cornelius Balbus in ancient Rome. Cornelius Balbo was not a Roman by birth but received Roman citizenship. This decision was contested, arguing that there are agreements prohibiting the granting of citizenship to members of certain tribes.

However, Balbo did not belong to any of these tribes 1 Balbus came from Gades in Spain. Gades, like Massilia or Saguntum, belonged to the so-called 'allied cities', which were outside of Italy. , so Cicero argued that since the law precisely sets exceptions when something is forbidden, it follows that in cases not covered by these exceptions, this is allowed.

To be honest, the translation should be rather “exception that checks the rule”, because the verb probare means beyond “to prove” and also “to check”. Thanks to this the wording makes more sense.