exception that proves the rule

The exception that proves the rule: this may very well be the most misused and misconstrued aphorism in existence. It is seemingly false on its face; an exception disproves rather than proves a rule. Where does the phrase come from and why do we say it?

The origins are in Latin legal maxim, exceptio probat regulam in casibus non exceptis (the exception confirms the rule in cases not excepted). In other words, the fact that an exception exists means that a general rule also exists, e.g., if you see a sign that says “No Parking on Sundays,” you have legal protection in assuming that parking is permitted the other six days. English use of the Latin proverb dates to at least 1640 when Gilbert Watts penned the following in his Bacon’s Advancement and Proficience of Learning:

As exception strengthens the force of a Law in Cases not excepted, so enumeration weakens it in Cases not enumerated.

A somewhat earlier use of the Latin in an otherwise English passage can be found in in Samuel Collins’s 1617 The Defence of the Bishop of Elie:

Indefinites are equivalent to vniversalls especially where one exception being made, it is plaine that all others are thereby cut off, according to the rule Exceptio figit regulam in non exceptis [the exception establishes the rule where not excepted].

Fowler’s 1926 classic manual of usage lists five different uses for this phrase. The first is the original, legal sense.

The second is a reinterpretation of the phrase where prove is used in the sense of to test, the exception tests the validity of the general rule. This usage is primarily found in scientific circles.

The third is a loose rhetorical sense in which the exception highlights and points out the general rule. A rainy day in Phoenix proves the rule that it is generally dry there.

Fowler’s fourth sense is the jocular nonsensical one. One uses the phrase in a deliberately nonsensical way for humorous or light-hearted purposes.

And the fifth sense is the result of unclear logic and sloppy writing.

(Sources: Oxford English Dictionary, 2nd Edition; The Yale Book of Quotations; Fowler’s Modern English Usage)

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