"The exception [that] proves the rule" is a frequently confused English idiom. The original meaning of this idiom is that the presence of an exception establishes that a general rule exists. Henry Watson Fowler's Modern English Usage provided an early critical analysis of the confused use of this phrase, and many more recent style guides repeat Fowler's criticism.
Fowler's five usages
Fowler's Modern English Usage identifies five ways in which the phrase is commonly used.
The phrase is derived from the medieval Latin legal principle exceptio probat regulam in casibus non exceptis ("the exception confirms the rule in cases not excepted"), a concept first proposed by Cicero in his defense of Lucius Cornelius Balbus. In other words, the fact that an exception is stated serves to establish the existence of a rule that applies to cases not covered by the exception. Fowler's Modern English Usage gives the following example:
"Special leave is given for men to be out of barracks tonight till 11.00 p.m."; "The exception proves the rule" means that this special leave implies a rule requiring men, except when an exception is made, to be in earlier. The value of this in interpreting statutes is plain.(*)
Similarly, 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).(**)
The phrase may also be invoked to claim the existence of a rule that usually applies, when a case to which it does not apply is specially mentioned. For example, the fact that a nurse is described as "male" (the exception) could be taken as evidence that most nurses are female (the rule). This is a slightly looser interpretation of the original meaning.(***)
A case may appear at first sight to be an exception to the rule. However, when we examine things more closely, we see that the rule doesn't apply to this case, so the rule is shown to be valid after all.
Fowler's example is of a critic, Jones, who never writes a favourable review. So we are surprised when he writes a favourable review of a novel by an unknown author. Then we discover that the novel is his own, written under a pseudonym. Obviously the rule doesn't apply to this case (although we may need to be more precise in stating the rule in future) and we can resume our previous opinion of Jones's ill-nature.
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.
"I am always punctual." "Were you on time for breakfast this morning?" "Well no, but the exception proves the rule, ha ha."
"It will rain on my birthday, it always does." "It didn't rain last year." "But the exception proves the rule."
Fowler writes "The last of these is the only one that need to be objected to directly, though 3 & 4 bear the blame of bringing 5 into existence." Fowler objects to the misuse of this proverb because it implies the following two beliefs:
- Exceptions can always be neglected. - 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."
(*)emphasis: "Special leave is given for men to be out of barracks tonight till 11.00 p.m.". Ergo: this special leave is not given for women.
(**)intended meaning: A sign that says "parking prohibited on Sundays" could serve as an indicator that people prevent, inhibit; thus prohibit, parking, for whatever reason, on that spot on Sundays - and for anyone willing to witness, or participate in, the event, that's the place and day to be.
(***)circumstances: "The fact that a nurse is described as "male"" serves as the indicator, for whatever reason, that the nurse in question is not female.
"If the rule is that the exception proves the rule, then what's the exception that proves that rule?"
Always keep in mind that the human brain can only comprehend 3 categories to put information in.