What’s the origin of the phrase ‘The exception that proves the rule’?
It’s a legal maxim, established in English law in the early 17th century. Written, as law was in those days, in Latin:
Exceptio probat regulam in casibus non exceptis
and is interpreted to mean ‘exception confirms the rule in the cases not excepted’
It has (slightly modified) examples in print going back to at least 1617:
Collins: Indefinites are equivalent to vniversalls especially where one exception being made, it is plaine that all others are thereby cut off, according to the rule Exceptio figit regulam in non exceptis.
While not the earliest citation, this, from Giovanni Torriano’s Piazza universale di proverbi italiani, or A Common Place of Italian Proverbs, 1666, expresses the idea clearly:
“The exception gives Authority to the Rule.”
See also: the List of Proverbs.