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