Etymology of Exception Proves the Rule

Native speakers of English accept without pause the many seemingly nonsensical sayings that are part and parcel of the vernacular. For instance, we blithely announce 'It's raining cats and dogs' to communicate the concept of a heavy downpour, yet spare precious little thought to the realization that neither felines nor canines play any part in the process of rainfall. For us, it's the saying's easy conveyance of meaning, not how it was put together, that matters. Another seemingly nonsensical familiar expression is 'The exception proves the rule.'
Etymology of Exception Proves the Rule