...doesn't mean that the exception is proof of the rule, since this would be self-defeating and idiotic (think: would more exceptions be greater proof of a rule?). The word "proves" is used in the sense of "tests" or "challenges" the rule, which makes sense, since an exception to a rule is an argument against its status as a rule, law, or similar attribution.
Next installment of "Intellectual Footprint OT" will be the expression, "(X) begs the question," which is frequently used so wrongly it is painful.
The RationalWiki page is damn good for this entry and makes a very good case for RaRa being WrongWrong, at least with respect to his thinking that the word "proves" is used in the sense of "tests" the rule - the wiki comes close to dismissing that Old English interpretation.
I also think they provided the best example seen, so far, of how the phrase was intended to be used: "For example, on a shop sign saying "we are open every day except
Christmas," the exception ("except Christmas") reinforces the rule ("we open every day") to be true in all cases other than that one stated exception."
The Latin interpretation
The original meaning of the phrase was that an exception to a rule demonstrates that there is a rule in order for it to be an exception. The full Latin phrase was
exceptio probat regulam in casibus non exceptis, meaning "the exception proves the rule to be true in cases not excepted." For example, on a shop sign saying "we open every day except
Christmas," the exception ("except Christmas") reinforces the rule ("we open every day") to be true in all cases other than that one stated exception. Another example would be a statement that implies an unspoken rule. For instance, "free parking on Sunday" obviously implies that one must pay to park on every other day of the week. This is a rule that is proved by its exception.
The existence of the same concept in multiple European languages supports this etymology. For example Spanish
"La excepción confirma la regla", French
"L'exception qui confirme la règle" and Romanian
"Excepția confirmă regula". Even non-Latin language German has "
Ausnahmen bestätigen die Regel". It would be really surprising if the Anglo-Saxon version outlined below had coincidently arrived at the same counter-intuitive statement but via a different route.
[edit]The Old English interpretation
The phrase is also sometimes, though with less reliability, claimed to mean "the exception
tests the rule," since the original meaning of "to prove" in Old English (originating from the Latin word
probat, compare
probe) was closer to "to test." This original use of "to prove" meaning "to test" can also be seen in the phrase "proof spirit" — which was
liquor which had been tested and shown to be of appropriate quality. This usage still exists — a site designated for testing equipment or weaponry may be called a "proving ground" (most notably in the
US Military). A firearm cartridge, or round, designed to produce pressures higher than what a firearm is rated for is called a "proof round" Firearms are required to be able to fire a "proof round" without failure to ensure they are safe to use with standard pressure rounds. A further example, used both metaphorically and literally, is the term
bulletproof.
In some cases, either of these definitions of "prove" create a working sentence, which somewhat explains the change in definition. For example I can say that
porn found on a
priest's hard drive "proves" that he is a
paedophile, and it works for either definition; it either (a) provides hard
evidence of paedophilia, thus proving the accusation to be true in the modern sense of the word, or (b) provides a legitimate test of whether or not the priest is a
paedophile — if the subjects of the porn are under age, he is defined as a paedophile. However, the aphorism "the exception that proves the rule" was not treated properly by the shift in definition, and now it looks like a way for idiots to
justify their idiocy.
Hence the phrase can be used correctly in the context of the
scientific method, when testing a
hypothesis by examining possible exceptions to it and whether they invalidate the hypothesis. So if I hypothesise that "all swans are white" (under the condition that at least one swan exists), the discovery of the
black swan (
Cygnus atratus) "proves" (meaning "tests") my rule. In this case, the rule is
falsified.
http://rationalwiki.org/wiki/Exception_that_proves_the_rule