Exception that proves the rule

The "exception that proves the rule", or similar variants of this aphorism, is an old adage that dates from a long time before the Internet laws it most resembles. It originates from the Latin phrase "exceptio probat regulam", attributed first to .The exception proves the rule (If there were no rule, no exception would be needed) by Opus the Penguin, alt-usage-english.org (archived from April 5, 2008).Pro Balbo, Caput XII by Marcus Tullius Cicero (archived from April 10, 2008). "Quod si exceptio facit ne liceat, ubi non sit exceptum, ibi necesse est licere.
You are most likely to encounter this phrase when somebody is speaking in generalisations or stereotypes and somebody else points out an example that clearly contradicts their comments. Retorting with the platitude that this is just the "exception that proves the rule" is an easy way of handwaving away this inconsistency.
There are some logical problems with this. While most kinds of rules have exceptions, and vague generalisations have more than most, the "exception that proves the rule" rhetoric glosses over why these exceptions exist. Any logical appraisal of a rule should regard it as weak, or even disproven, if cases can be found where it does not apply, unless there is a clear reason why it should not be applied in these cases. Instead, these exceptions are said to prove the rule, suggesting paradoxically that the truth of a rule is somehow strengthened by not always being true.
==Wait… why is this even an aphorism?==
There is no real academic linguistic debate over the origin of the phrase. There are two positions: one from the original Latin, and another based on "folk linguistics" stemming from the Old English meaning of the word "prove".
===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 proven 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 coincidentally arrived at the same counter-intuitive statement but via a different route.
===The Old English interpretation===
The phrase is also sometimes, though almost certainly incorrectly (see above), 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 contain the claimed proportion of alcohol. 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, which came from the late-sixteenth-century practice of firing a pistol at armor to prove it could stand up to a bullet. Similarly, homeopathy uses this sense of the word for proving, though there is neither testing nor proving in the scientific sense.
In some cases, either of these definitions of "prove" creates 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.
==Logically valid (mis)uses==
Because the phrase "exception that proves the rule" is wedged in the English language, it gets used for many situations. A few of them are even logically valid. Here are a few of those marginally logically valid uses not covered by the above:
* The fact that the current example is an exception proves the rule. 
** For example: "Townsville is usually a quiet place." "What about the Jazz festival? It's really loud then." "Well, that's the exception that proves the rule." 
** Differs slightly from the Latin case above, in that rather than the exception making the rule "stronger" when not in the exception, the fact that there are a finite number of exceptions (usually exactly one) means the rule is generally true, rather than completely so.
* The exception shows why the rule exists, or the logic behind the rule. 
** For one example: "...[Music critic] hates Arena Rock acts." "But he likes Queen!" "Queen's the exception that proves the rule: He regularly bashes their more mainstream-aiming stuff, and praises their more Artsy-Fartsy phases." (In other words: He has consistent taste, it's just that this one band also exists both inside and outside of the genre he dislikes.)
** For another example: "Freeways in [country] should have middle shoulders of at least 2.5 meters between opposing directions. The exception being [bad highway], which was grandfathered in, as it was already built when the rule was formulated; note, however, that it has 70% more accidents involving cross-traffic than any other freeway, thereby making it the exception that proves the rule." (In other words: The rule is there for a reason. The exception shows that reason.)
** "If you were to ask people who owned [product], 'do you use it for anything besides entertainment?', the few answering 'yes' will, upon examination, turn out to be exceptions that prove the rule: their utilitarian use will always turn out to have another, cheaper alternative product that can fill the same need better." (In short: The "exceptions" turn out not to actually be exceptions upon further examination.)
** Differs from the Old English case in that this is not a test: The rule is held to be correct, but either the rule or the exception is more detailed than is usually summarized.
==See also==
*Cognitive dissonance
*Common sense
*Falsifiability
*Hypocrisy
*No True Scotsman
*Special pleading
==References==
Category:Informal fallacies
Category:Latin phrases
Category:Rhetoric