Gautham and I were recently having a discussion about the phrase "The exception that proves the rule". I have steadfastly maintained that this phrase is inherently nonsensical, and I haven't had anybody offer up a good explanation. My thought was perhaps one could use it when you have a situation where you have something that looks like an exception to the rule, but if you look at it a bit more closely, it turns out that the exception arises from some mistaken assumption that proves the rule. Like:
Rule: "All rubber ducks are yellow."
Exception: "Look, this rubber duck is green!"
Exception that proves the rule: "That's actually a plastic duck."
Rule: "All scientists are dorks."
Exception: "What about Russell Crowe in A Beautiful Mind?"
Exception that proves the rule: "Russell Crowe is actually an actor. He is also reportedly a complete jerk (based on his interactions with real mathematicians during the making of the film). But he's not a dork."
But it usually isn't used that way. Mostly, it seems that it's used as a (feeble) retort to an exception to somebody's going theory. Usually because the exception is not really an exception or the rule is not really a rule:
Rule: "Scientists are dorks."
Exception: "Richard Feynman wasn't a dork."
Exception is not an exception: "That's an exception that proves the rule. Richard Feynman is also a dork."
Exception guy: "?"
Rule: "All computer scientists have long hair in ponytails."
Exception: "What about this dude?"
My rule is wrong: "..." [couple seconds] "That's just the exception that proves the rule!"
Well, it turns out that Wikipedia has a pretty solid discussion of the topic. Turns out the idea behind this saying is really more like "The exception proves the existence of an rule." Wikipedia's example is a sign that says "Parking prohibited Sundays" is an exception that proves the existence of the rule that parking is generally allowed. Apparently this comes from ancient Roman law! Who would have known?
One thing I find amazing about this is that in the old days pre-Internet, probably only a few scholars who read books and stuff would know this fact. And those guys would seem so smart just by virtue of knowing some obscure fact. But now everyone can know those obscure facts. All you have to do is have the question, and someone out there probably has the answer for you. Cool. Now everyone can be learned. Which reminds me of this exchange from The Simpsons.