Exception paradox: if every rule has an exception, then there must be an exception to the rule that every rule has an exception. From the logical point of view, this can be taken as a proof that the sentence “every rule has an exception” is false - a simple example of a proof technique known as reductio ad absurdum. More formally, Every rule has an exception. (Statement) “Every rule has an exception” has an exception. (By 1) There exists some rule R without exception. (By 2) Since R is a rule, by the first statement it must have an exception. But by 3, it does not have an exception - a contradiction. Read more at en.wikipedia.org

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