The exception proves the rule.
English has the noun 'proof', the verb 'to prove', and another verb 'to
proof'. These all arise from a common Germanic form: compare for instance
German 'prüfen' (to test), Norwegian 'å prøve' (to test), Danish 'at prøve'
(to test), Swedish 'at pröve' (to test). There is also an Icelandic form
(which I can't remember off the top of my head), and an Anglo-Saxon form
(which I also can't remember right now). But anyway, all of these forms mean
'to test', as opposed to 'to have tested and have the proof ready to hand'.
However, a weird thing happened with the English forms of the Germanic stem.
The three principal forms split themselves between two different meanings.
Two assumed a meaning of 'this has been proven, and I'm now showing you the
proof, buddy', as in ...
'to prove': to show that something is correct, on the basis of
proof that you have
'proof': to have information that shows that something is correct
... but one English form retained the meaning of 'to test, to
undertake a test', rather than to have already tested and have the
proof at hand, as in ...
'to proof': to examine and test (usually a piece of text) for
mistakes, falsehoods, etc.
It is only in this one form, 'to proof' that the real Germanic meaning
survives. It is also with this form that the phrase 'The exception proves
the rule should be read'. It really means that the exception is testing
whether the rule is valid or not. A re-phrasing should really give 'The
exception tests the rule', or even 'The exception proofs the rule', if you
expand the meaning of 'to proof' beyond the correction of texts.
Germanic is such fun ...
This seemed perfectly reasonable to us, especially since it confirmed
Vin's suspicions. But then we got a second opinion from my personal
linguist Ben Bergen:
English 'prove' is a borrowing from Latin through Old French and
Anglo-Norman. The American Heritage Dictionary gives the following
etymology (Webster gives essentially the same):
[Middle English prove, preve, from Anglo-Norman prove, and from Old French
prueve, both from Late Latin proba, from Latin probre, to prove; see
prove.]
The expression 'the exception proves the rule' appears to be borrowed
directly from Latin. Interestingly, it seems NOT to be that prove
originally had the meaning 'test', despite the fact that 'proof' does mean
'test' in expressions like 'proof reader', 'the proof is in the pudding',
and the like.
The Dictionary of Modern English Usage cites the original legal sense of
'the exception proves the rule' as follows:
"'Special leave is given for men to be out of barracks tonight till 11.0
p.m.'; 'The exception proves the rule' means that this special leave
implies a rule requiring men, except when an exception is made, to be in
earlier. The value of this in interpreting statutes is plain."
The phrase seems to date from the 17th century. Anthony Cree, in _Cree's
Dictionary of Latin Quotations_ (Newbury, 1978) says that the phrase comes
from classical Latin (Latin spoken before A.D. 400). In full, the Latin
seems to be 'exceptio probat regulam in casibus non exceptis.' ('The
exception proves the rule in cases not excepted.') Or in a shorter
version: 'exception probat regulam'.
As for the Germanic cognates, I'm not a Germanicist, so I can't speak to
them.
So that's the story so far. I'm not sure the matter has been conclusively settled (informed input welcome!).
More importantly, the following has been at the top of my whiteboard since September 2000:
NC is the exception to the rule that the exception proves the rule that proves it.
Nancy is the exception that proves that the exception proves the rule.