exceptio probat regulam de rebus non exceptis
Latin phrase
ex·cep·tio pro·bat re·gu·lam de re·bus non ex·cep·tis eks-ˈkep-tē-ˌō-ˌprō-bät-ˈrā-gu̇-ˌläm-dā-ˈrā-ˌbu̇s-ˌnōn-eks-ˈkep-ˌtēs 
: an exception tests the rule as to things not excepted
also : an exception can demonstrate that the rule exists