[blml] Exceptio probat regulam in casibus non exceptis [SEC=UNOFFICIAL]

[Nigel]

[Richard Hilss Quotes 
Michael Quinion, World Wide Words, Exception That Proves The Rule]

http://www.worldwidewords.org/qa/qa-exc1.htm

[snip]

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.

[snip]

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.....

{nige1]
Excellent stuff, Richard :) 
The educational value of BLML is much enhanced by literary and philosophical quotations from Grattan Endicott, David Burn, yourself and others :)  

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