EXCEPTION THAT PROVES THE RULE
[Q] From Dave Dewhurst, British Columbia: “How did the phrase, the exception that proves the rule come about? Would an exception to some pattern or consistency not prove the need for a rule, not the existence of one?”
[A] You’re right to query the expression. It has caused as much confusion as any other in the language and is often argued about. The misunderstanding has been amplified by well-meaning but incorrect attempts going back a century to explain it.
These days it is often used sweepingly to justify an inconsistency. Those who use it seem to be saying that the existence of a case that doesn’t follow a rule proves the rule applies in all other cases and so is generally correct, notwithstanding the exception. This is nonsense, because the logical implication of finding that something doesn’t follow a rule is that there must be something wrong with the rule. As the old maxim has it, you need find only one white crow to disprove the rule that all crows are black.
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” or “the proof of the pudding is in the eating”, or in the printer’s proof, which is a test page run off to see that all is correct with the typesetting). 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. If you can’t reconcile the supposed exception with the rule, there must indeed be something wrong with the rule. The expression is indeed used in this sense, but that’s not where it comes from or what it strictly means.
The problem with that attempted explanation is that those putting it forward have picked on the wrong word to challenge. It’s not a false sense of proof that causes the problem, but exception. We think of it as meaning some case that doesn’t follow the rule, but the original sense was of someone or something that is granted permission not to follow a rule that otherwise applies. 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”.
Let us say that you drive down a street somewhere and find a notice which says “Parking prohibited on Sundays”. You may reasonably infer from this that parking is allowed on the other six days of the week. A sign on a museum door which says “Entry free today” leads to the implication that entry is not free on other days (unless it’s a marketing ploy like the never-ending sales that some stores have, but let’s not get sidetracked). H W Fowler gave an example from his wartime experience: “Special leave is given for men to be out of barracks tonight until 11pm”, which implies a rule that in other cases men must be in barracks before that time. So, in its strict sense, the principle is arguing that the existence of an allowed exception to a rule reaffirms the existence of the rule.
Despite the number of reference books which carefully explain the origin and true meaning of the expression, it is unlikely that it will ever be restored to strict correctness. The usual rule in lexicography is that sayings progress towards corruption and decay, never the reverse. Unless this one proves to be an exception ...
http://www.worldwidewords.org/qa/qa-exc1.htm
Close enough for gummint work, eh?? But seriously, as you can see from the explanation (and thanks for posting) the common interpretation is incorrect--and THAT was my point.