the exception proves the rule
1. The existence of an exception to a supposed rule proves the general truth of the rule (often used in argument when no such conclusion is justified)
2. A distorted translation of a part of a legal Latin phrase meaning ‘the making of an exception proves that the rule holds in cases not excepted’
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Main Entry: ↑except
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the exception proves the rule
Generally held to mean that the finding or making of an exception proves that the rule holds good for all cases that are not exceptions, but held by some to mean that the exception tests the rule, thereby proving its general validity
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Main Entry: ↑prove
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proverb the fact that some cases do not follow a rule proves that the rule applies in all other cases
Origin:
From Latin exceptio probat regulam in casibus non exceptis ‘The exception confirms the rule in the cases not excepted’. The exception here is ‘the action of excepting’ not ‘that being excepted’. By specifically excluding cases where the rule doesn't apply, you make the rule stronger for the cases still governed by it
Useful english dictionary. 2012.