Exception that proves the rule
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.
The first, preferred by Fowler, is that the presence of an exception applying to a specific case establishes ("proves") that a general rule exists.
[6] This meaning of the phrase, which for Fowler is the original and clearest meaning,[1] is thought to have emerged from the legal phrase "exceptio probat regulam in casibus non exceptis" ("the exception proves the rule in cases not excepted"),[7] an argument attributed to Cicero in his defence of Lucius Cornelius Balbus.
The value of this in interpreting statutes is plain.In other words, under this meaning of the phrase, the exception proves that the rule exists on other occasions.
Fowler describes two versions of this use, one being the "loose rhetorical sense" and the other "serious nonsense";[1] other writers connect these uses together insofar as they represent what Holton calls a "drift" from the legal meaning.
Under this sense it is "the unusualness of the exception"[2] which proves how prevalent the tendency or rule of thumb to which it runs contrary is.
[1] The previous evaluation of Jones's ill-nature toward others is re-affirmed by discovering the manner in which the exception falls outside the rule.
Holton argues that this origin involves a "once-heard etymology" which "makes no sense of the way in which the expression is used.
[2] Nonetheless, it does for Fowler pass the test of making grammatical sense[1] and it is also referenced as a possible meaning within the Oxford English Dictionary.
[7] In any case, the phrase can be interpreted as a jocular expression of the correct insight that a single counterexample, while sufficient to disprove a strictly logical statement, does not disprove statistical statements which may correctly express a general trend notwithstanding the also commonly encountered existence of a few outliers to this trend.
