Posted by Eugene Volokh:
"The Exception Proves the Rule":
http://volokh.com/archives/archive_2009_05_03-2009_05_09.shtml#1241808692
People often find the origin of this phrase puzzling -- wouldn't an
exception disprove the rule? Some have argued that "proves" was used
in its meaning "To make trial of; to try, test" (definition II in the
Oxford English Dictionary).
But as best I can tell, the origin of the phrase is the legal
principle that the statement of an exception shows that the rule is
opposite in the cases not excepted. Here's what the OED tells us:
The legal maxim, �Exception proves (or confirms) the rule in the
cases not excepted� (exceptio probat regulam in casibus non
exceptis), which is in its original form an example of sense 1, is
commonly quoted as �The exception proves the rule� ....
Here's an early English American case, Watson v. Alexander, 1 Va. 340
(1794): "The act excepts the cases of contracts for gold or silver
coin, tobacco, or other specific property; and if it be true, that an
exception proves the rule, we must decide that all other contracts are
within the law." And here's an illustration given by the OED that
shows the meaning beyond the legal context; it's from Samuel Johnson's
preface to a 1765 edition of Shakespeare (emphasis added):
It has been much disputed, whether Shakespeare owed his excellence
to his own native force, or whether he had the common helps of
scholastick education, the precepts of critical science, and the
examples of ancient authours.
There has always prevailed a tradition, that Shakespeare wanted
learning, that he had no regular education, nor much skill in the
dead languages. Johnson, his friend, affirms, that he had small
Latin, and no Greek; who, besides that he had no imaginable
temptation to falsehood, wrote at a time when the character and
acquisitions of Shakespeare were known to multitudes. His evidence
ought therefore to decide the controversy, unless some testimony of
equal force could be opposed.
Some have imagined, that they have discovered deep learning in many
imitations of old writers; but the examples which I have known
urged, were drawn from books translated in his time; or were such
easy coincidences of thought, as will happen to all who consider
the same subjects; or such remarks on life or axioms of morality as
float in conversation, and are transmitted through the world in
proverbial sentences....
There are a few passages which may pass for imitations, but so few,
that the exception only confirms the rule; he obtained them from
accidental quotations, or by oral communication, and as he used
what he had, would have used more if he had obtained it.
The Comedy of Errors is confessedly taken from the Menæchmi of
Plautus; from the only play of Plautus which was then in English.
What can be more probable, than that he who copied that, would have
copied more; but that those which were not translated were
inaccessible?
By "the exception confirms the rule," Johnson seems to have meant that
"seeing the exception, and recognizing that it is an exception,
confirms for us that there is a rule." And my sense is that in the
modern day, the phrase -- when used effectively -- has roughly that
meaning.
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