MemeMachineGo!

‘The exception that proves the rule’ turns out to have a source in law. And here I thought it was just an annoying bit of sophistry.

Alan Bliss, in A Dictionary of Words and Phrases in Current English, has the following to say about the origin of this phrase: “Exception probat regulam [Lat.], the exception proves the rule. A legal maxim of which the complete text is: exceptio probat [or (con)firmat] regulam in casibus non exceptis— ‘the fact that certain exceptions are made (in a legal document) confirms that the rule is valid in all other cases.’”

The application is this. Suppose a law is stated in such a way as to include an exception, e.g., “Parking is prohibited on this street from 7 AM to 7 PM, Sundays and holidays excepted.” The explicit mention of the exception means that NO other exceptions are to be inferred. Thus we should take the Latin verb probare in the maxim to have the sense of “to increase the force of.”