Snopes explains. Aargh, just found out I'd been wrong about this all these years too.

Some who think they're in the know attempt to explain the seeming contradiction of 'The exception proves the rule' by looking at the verb used in the aphorism in a less common fashion. While we're now most familiar with 'proves' as a verb meaning 'to establish as truth,' an older meaning of that self-same word that has since almost disappeared from everyday usage defines it as meaning 'to test.' By those lights, 'The exception proves the rule' should be read as 'The exception tests the rule' — that is, the contradiction puts the claim through its paces and finds it wanting.


Yet even that is not the case. Our puzzling saying is actually a legal maxim drawn from early 17th century English law. It was then written in Latin as Exceptio probat regulam in casibus non exceptis, which translates into English as Exception confirms the rule in the cases not excepted. More simply, 'The exception proves the rule exists' — the fact that certain exceptions are made in a legal document or announcement confirms the rule is in force at all other times.

Right, TheophileEscargot. This is even more evident when dealing with norms-- "unwritten" rules rather than legal codes. The rule is never more evident than when it's been breached; indeed the breaching of the rule helps us to understand that the rule exists.

For what it's worth, whole episodes -- whole seasons -- of Seinfeld were predicated on testing this premise. Many of us didn't even really realize seriousness of the unwritten rules about personal space, or snack food etiquette, until faced with the Close Talker or the Double Dipper. The exception, and the pique that the exception raised, proves the existence of the rule.

Damn, .kobayashi, busting out some Seinfeld to drop knowledge! I have a friend who does that and it never ceases to amaze me that I under-appreciated the genius of Seinfeld all these years.

More simply, 'The exception proves the rule exists' — the fact that certain exceptions are made in a legal document or announcement confirms the rule is in force at all other

When a baker proofs yeast, s/he's testing it to be sure it's still good. That's how I've always understood "proofs/proves" to meant in "proves the rule," i.e., testing it to be certain it's still useful and sound.

OK, this is really helpful. I can totally agree with 'the exception proves the rule exists'. But I think people must use this phrase wrong because I've heard it in cases like, "Dogs always chase cats. Except our dog loves cats. The exception proves the rule!" (Silly example, but illustrates how it seems to me people use this in a meaningless way.) In some cases, an exception does not prove the existence of a rule, it calls into question the idea that there is a rule!

Yeah, but.... look at it this way: The fact that someone would think that a dog that loves cats would be highly unusual is what proves the rule. The rule, then, is the usual situation.

Our dog, incidentally, loves teasing our cat. And the cat loves teasing the dog. But, strangely, they seem to tease each other with a certain measure of affection.

Weird, huh?

... I've always understood "proofs/proves" to meant in "proves the rule," i.e., testing it to be certain it's still useful and sound.
I've always thought it meant this, too.