http://en.wikipedia.org/wiki/Exception_ ... s_the_rule
Exception that proves the rule
The phrase is derived from the medieval Latin legal principle exceptio probat regulam in casibus non exceptis ("the exception confirms the rule in cases not excepted"), a concept first proposed by Cicero in his defense of Lucius Cornelius Balbus. In other words, the fact that an exception is stated serves to establish the existence of a rule that applies to cases not covered by the exception. Fowler's Modern English Usage gives the following example:
Similarly, 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)."Special leave is given for men to be out of barracks tonight till 11.00 p.m."; "The exception proves the rule" means that this special leave implies a rule requiring men, except when an exception is made, to be in earlier. The value of this in interpreting statutes is plain.
The phrase may also be invoked to claim the existence of a rule that usually applies, when a case to which it does not apply is specially mentioned. For example, the fact that a nurse is described as "male" (the exception) could be taken as evidence that most nurses are female (the rule). This is a slightly looser interpretation of the original meaning.