THE EXCEPTION THAT PROVES THE RULE….A reader takes Andrew Sullivan to task today for using the expression “the exception that proves the rule.” As this link explains, there are at least two possible ways in which modern day usage has corrupted the original meaning of the phrase:

It has often been suggested in reference works that prove here is really being used in the sense of “test” (as it does in terms like “proving ground”)….It is said that the real idea behind the saying is that the presence of what looks like an exception tests whether a rule is really valid or not.

….[But] it’s not a false sense of proof that causes the problem, but exception….The true origin of the phrase lies in a medieval Latin legal principle: exceptio probat regulam in casibus non exceptis, which may be translated as “the exception confirms the rule in the cases not excepted”….A sign on a museum door which says “Entry free today” leads to the implication that entry is not free on other days.

This is good stuff for us pedant types. I’ve always bought into the “proving ground” explanation myself, but the second explanation really does sound more plausible. This is the first time I’ve heard it.

But here’s a question: how did the phrase get corrupted in the first place? I think it must satisfy a deep human desire to avoid admitting error. After all, its current usage is so obviously absurd (an exception to a rule proves the rule is true?) that it wouldn’t manage to stick around unless it satisfied some highly desirable rhetorical market niche. And it does: even in the face of indisputable evidence of error (“Actually, George Bush was shorter than John Kerry and he won anyway”) it provides a snappy comeback (“He’s the exception that proves the rule!”) that leaves your average know-it-all windbag gasping (“Huh?”). Victory is yours! Nonsensical or not, that makes it a pretty handy handy phrase to have around, doesn’t it?