"
The exception that proves the rule" (sometimes "
the exception proves the rule") is a saying whose meaning is contested.
Henry Watson Fowler's
Modern English Usage identifies five ways in which the phrase has been used,
[1] and each use makes some sort of reference to the role that a particular case or event takes in relation to a more general rule.
Two original meanings of the phrase are usually cited. The first, preferred by Fowler, is that the presence of an exception applying to a
specific case establishes ("proves") that a
general rule exists. A more explicit phrasing might be "the exception that proves
the existence of the rule."
[1] Most contemporary uses of the phrase emerge from this origin,
[2] although often in a way which is closer to the idea that all rules have their exceptions.
[1] The alternative origin given is that the word "prove" is used in the archaic sense of "test".
[3] In this sense, the phrase does not mean that an exception demonstrates a rule to be true or to exist, but that it tests the rule, thereby proving its value. There is little evidence of the phrase being used in this second way.