‘The exception [that] proves the rule‘ is a saying whose meaning has been interpreted or misinterpreted in various ways. Its true, or at least original, meaning is that the presence of an exception applying to a specific case establishes (‘proves’) that a general rule exists.
For example, a sign that says ‘parking prohibited on Sundays’ (the exception) ‘proves’ that parking is allowed on the other six days of the week (the rule). A more explicit phrasing might be ‘the exception indicates the existence of the rule.’
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The Daily Omnivore 