An advertisement for “Say What You Mean! A Troubleshooter’s Guide to English Style and Usage” by R.L. Trask included this example from the book:
“EXCEPTION THAT PROVES THE RULE, THE — The sense of this expression is often lost. What it means is this: having an exception to the rule proves there is a rule in the first place.”
I used to agree with that interpretation, until I actually thought about it. If an exception proves the existence of a rule, do two or three or four exceptions prove it even more conclusively? Or, on the contrary, do a bunch of exceptions prove there is no rule worth paying attention to?
Once, I believed I’d gained enlightenment from The Penguin Dictionary of American English Style and Usage:
“As it is commonly used, this cliche [the exception that proves the rule] contradicts scientific logic. Yet those invoking it assume that it bolsters their points of view. … A TV commentator stated a rule that caucus winners lose elections and added, ‘Jimmy Carter, the exception that proves the rule, won Iowa in 1976.’
“How can an exception ‘prove’ a rule? Shouldn’t the exception disprove it? The answer lies in an archaic meaning of prove: to test … A scientist may test a rule (or theory, principle, etc.) by seeking an exception; finding one is reason to discard, or at least modify, the rule. Users of the cliché, unaware of its true meaning, cite it as authority to keep the rule despite the exception.”
If I’d stopped there, I could have been happy, I think. Rashly, I consulted yet another source, Garner’s Dictionary of Modern American Usage: “This phrase is the popular rendering of what was originally a legal maxim, ‘The exception proves (or confirms) the rule in the cases not excepted’ (exception probat regulam in casibus non exceptis). Originally exception in this maxim meant ‘the action of excepting’ – not, as is commonly supposed, ‘that which is excepted’ – so that the true sense of the maxim was that by specifying the cases excepted, one strengthens the hold of the rule over all cases not excepted.”
Garner says there are at least two “spurious” explanations of the phrase. “One is that because a rule does not hold in all instances (i.e., has exceptions), the rule must be valid. … A more sophisticated, but equally false, explanation of the phrase is that prove here retains its Elizabethan sense (derived from the Latin) ‘to test’ … ”
Even H.W. Fowler’s classic Dictionary of Modern English Usage can’t help. Fowler’s page-long entry is, as Garner warned, “all but incomprehensible.” I think I’ll just avoid using exception that proves the rule. That’s a sound approach to cliches generally, come to think of it.