Exception that proves the rule

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"The exception that 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] a reading advocated, for example, by a 1918 Detroit News style guide:

The exception proves the rule is a phrase that arises from ignorance, though common to good writers. The original word was preuves, which did not mean proves but tests.^[4]

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.^[1][2][5]