Proving the rule by the exception

     Val frowned. "Aunt Di," he said, "was what Uncle Lionel's brother Adolphe used to call a freak of nature. I remember him saying to me, 'Val, my boy, you never get a woman who is a complete fool. Many men achieve that distinction, but never a woman. The exception which proves that rule is your Aunt Diana.' He didn't like her."

(from The Gyrth Chalice Mystery, by Margery Allingham)