Consider the oft-quoted statement "the exception proves the rule." Most people take this to mean that the exception confirms the rule, though when you ask them to explain the logic in that statement, they usually cannot. After all, how can an exception prove a rule? It can't. The answer is that an earlier meaning of prove was to test {a meaning preserved in proving ground} and with that meaning the statement suddenly becomes sensible-the exception tests the rule.
( Bill Bryson )
[ The Mother Tongue: English and ]
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