From the legal principle
exceptio probat regulam in casibus non exceptis ("the exception confirms the rule in cases not excepted").
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.
e.g. 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).
http://en.wikipedia.org/wiki/Exception_that_pr oves_the_rule