Q: Can you help me understand how an exception can prove a rule? I’ve often heard it said that this expression made sense at one time when the word “prove” meant to test rather than to confirm absolutely. Is that correct?
A: The old saying “the exception that proves the rule” does seem nonsensical. If there’s an exception, then it should disprove the rule, right? Many word lovers have turned themselves inside out in an attempt to explain this seeming contradiction.
But the word “proves” isn’t the key to the problem. (Contrary to statements in several reference books, “proves” here does indeed mean proves, not tests.) The key is the word “exception,” which English adopted from French in the 14th century.
When the word (spelled excepcioun) showed up in Chaucer’s writings in 1385, it meant a person or thing or case that’s allowed to vary from a rule that would otherwise apply.
That sense of the word led to the Medieval Latin legal doctrine exceptio probat regulam in casibus non exceptis (exception proves the rule in cases not excepted), according to the Oxford English Dictionary.
By the 17th century, the Latin expression was being quoted in English as “the exception proves the rule” or variations on this. And the exception, the OED tells us, was something that “comes within the terms of a rule, but to which the rule is not applicable.”
If all students in a school are required to attend gym class, for example, that’s the rule. If a kid with a sprained ankle is excused from gym, then the exception made for him proves that there’s a rule for everybody else.