The Exception Proves the Rule

All models are wrong

The exception [that] proves the rule‘ is a saying whose meaning has been interpreted or misinterpreted in various ways. Its true, or at least original, meaning is that the presence of an exception applying to a specific case establishes (‘proves’) that a general rule exists.

For example, a sign that says ‘parking prohibited on Sundays’ (the exception) ‘proves’ that parking is allowed on the other six days of the week (the rule). A more explicit phrasing might be ‘the exception indicates the existence of the rule.’

An alternative explanation often encountered is that the word ‘prove’ is used in the archaic sense of ‘test.’ Thus, the saying does not mean that an exception demonstrates a rule to be true or to exist, but that it tests the rule. For example, Mutillidae are wasps without wings which cannot fly, and therefore are an exception that proves (tests) the rule that wasps fly. The explanation that ‘proves’ really means ‘tests’ is, however, is not universally accepted.

Lexicographer Henry Watson Fowler’s ‘Modern English Usage’ identifies five ways in which the phrase is commonly used, listed in order from most to least correct: Original meaning, Scientific,  Loose rhetorical, Jocular, and Serious.

The original meaning of the phrase is derived from a legal principle of republican Rome: ‘exceptio probat regulam in casibus non exceptis’ (‘the exception confirms the rule in cases not excepted’), a concept first proposed by Roman Statesman Cicero in his defence of Iberian Roman Lucius Cornelius Balbus who in 56 BCE was prosecuted for illegally assuming the rights of a Roman citizen. The original meaning illustrates that a stated exception implies the existence of a rule to which it is the exception. The second part of Cicero’s phrase, ‘in casibus non exceptis’ or ‘in cases not excepted,’ is almost always missing from modern uses of the statement that ‘the exception proves the rule,’ which may contribute to frequent confusion and misuse of the phrase.

This legal principle is classically referred to as ‘inclusio unius est exclusio alterius’ (‘Inclusion of one is to exclude the others’). The idea is that if the promulgator of law finds reason to enumerate one exception, then it is only reasonable to infer no others were intended. The Ninth Amendment of the United States Constitution was enacted to explicitly suppress this principle by stating that ‘The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.’

The second of Fowler’s definitions, the scientific sense, is based on the observation that a case may appear at first sight to be an exception to the rule. However, when the situation is examined more closely, it is observed that the rule does not apply to this case, and thus the rule is shown to be valid after all. Fowler’s example is of a critic, Jones, who never writes a favorable review. So it is surprising when he writes a favourable review of a novel by an unknown author. Then it is discovered that the novel is his own, written under a pseudonym. Obviously the rule doesn’t apply to this case (although the rule may need to be more precisely stated in future) and the previous evaluation of Jones’s ill-nature toward others is reaffirmed.

An example of this use in science writing is laid out by evolutionary biologist Richard Dawkins in ‘The Ancestor’s Tale.’ Cnidaria is a phylum of animals including jellyfish, corals, and sea anemones. The rule is that all cnidarians, and only cnidarians, have specialized harpoon cells called ‘cnidocytes,’ which they often use to capture and/or inject venom into prey. There is one exception to this rule. Some species of sea slugs of the nudibranch group have tentacles containing cnidocytes, even though the slugs aren’t cnidarians. But it turns out that the slug eats jellyfish and passes the jellyfish’s commandeered weapons, intact and still working, into its own tentacles. So examining the only known exception really proved the original rule valid after all.

In an example of the third definition, the loose rhetorical sense, a rural village is ‘always’ quiet. A local farmer rents his fields to a rock festival, which disturbs the quiet. In this example, saying ‘the exception proves the rule’ is literally incorrect, but it is used to draw attention to the rarity of the exception, and to establish the status of the village prior to the exceptional event. The general misuse of the phrase is attributable to the ambivalence of the word ‘rule.’ In the original sense, ‘rule’ is taken as a strict rule, while in the loose rhetorical sense ‘rule’ is taken to mean ‘rule of thumb.’

The fourth definition, the loose rhetorical sense, refers to usages where the original sense of the phrase could only apply to a situation if it were somehow altered, for example: if it is common place for a nurse that is male to be described as ‘a male nurse,’ it could be taken as evidence to a rule of thumb that most nurses are female. The fifth definition, the jocular sense, refers to usages where a speaker is aware that a phrase does not correctly apply to his or her initial statement, but is appealing to it ironically. For example, if they make the statement ‘I am always punctual,’ and are then asked, ‘Were you on time for breakfast this morning?’ and then respond, ‘Well no, but the exception proves the rule.’

The final, and least authoritative definition, the serious sense, refers to situations where the original speaker has confused the meaning of the phrase and uses it literally, such as if they say, ‘It will rain on my birthday, it always does,’ only to be told that it didn’t rain last year and then to counter that the exception proves the rule. The first speaker in this example apparently believes that any exception to any rule ‘proves’ the rule true; in this case, the notion that ‘the exception proves the rule in cases not excepted’ is neither implied nor understood by the speaker.

Fowler writes ‘The last of these is the only one that need to be objected to directly, though three and four bear the blame of bringing five into existence.'” Fowler objects to the misuse of this proverb because it implies the following two beliefs: Exceptions can always be neglected and a truth is all the truer if it is sometimes false. It was in objection to this misuse that Arthur Conan Doyle had his famous detective Sherlock Holmes utter the statement ‘I never make exceptions. An exception disproves the rule.’

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