Exceptions, of course, disprove rules. Despite this, many people confronted by a counter-example to their claim will dismiss it as ‘the exception that proves the rule’. The fallacy consists in the dismissal of a valid objection to the argument.
‘You never find songs written about any towns in Britain apart f London. ‘
‘What about “Scarborough Fair?” ‘
‘That’s the exception that proves the rule. ‘
(If one leaves Liverpool and Old Durham Town out of it.)
The origin of the fallacy lies in the changing uses of language. The word ‘prove’, which is now taken to refer to establishing something beyond doubt, used to mean ‘test’. Something would be ‘proved’ to establish its quality, and this is the sense which has passed down to us in this fallacy. The exception puts the rule to the test and, if it is found to be a valid exception, refutes it instead of proving it in the modern sense of the word:
No fictional character ever attracted fan clubs in distant countries like pop stars do. Sherlock Holmes does, of course, but he’s simply the exception that proves the rule.
(An elementary fallacy, dear Watson.)
There is a very loose way in which an exception can help to point to an otherwise general truth. If we all recognize an exception as remarkable, and identify it as such, then it does show that we accept that the rule which it counters does usually apply. In this sense, the one case we recognize as a freak points to the otherwise general truth:
Medical advances are made by painstaking research, not by chance. know there was penicillin, but everyone knows that was a chance in a million.
(Whether true or not, this is a legitimate line of argument provided the rule is not claimed as universal. Everyone’s acknowledgement of the unique exception points to a rule which says the opposite, with this one exception.)
Even in this specialized case, the exception disproves the universal rule. The trouble with sweeping statements is that it really does take only one exception to negate them. The medieval world abounded with universal which assured people that the sun would always rise and set each day, and that there could be no such thing as a black swan. A visit to the land of the midnight sun scuppered the first one, and the discovery of black swans in Australia polished off the second. It would be pleasant for many people if they could live in a world of certainties, surrounded by huge general truths. Exceptions come baying at that cozy world like wolves at the fringes of a camp-fire. They introduce uncertainties and doubts, and the temptation there is to use the fallacy quickly to get rid of them so that we can continue as before.
The exception that proves the rule is a fallacy beloved of those who are emphatic in their judgements. They have the world neatly divided into categories, and do not intend the irritant sand of exceptions to intrude into the well-oiled machinery of their worldview. In their smooth-running world, all pop stars are drug addicts, all feminists are lesbians and all young people are weirdos. Any honourable exceptions to the above categories are ejected with equal smoothness as ‘exceptions that prove the rule’. The great thing about this particular fallacy is that it renders your argument invulnerable to factual correction. The most embarrassing proofs that you are just plain wrong can be swallowed whole as ‘exceptions that prove the rule’, and need occasion no more than a slight pause in your declamation.
‘Lend us a fiver. I’ve always paid you back before.’
‘What about last week?’
‘That was the exception that proves the rule. You know you’ll get it back in the long run. ‘
(Put on your trainers.)
The above article is from the book How To Win Every Argument by Madsen Pirie. The article is only for educational and informative purposes to explain and understand formal logic and logical fallacies. It is a great book, definitely worth a read!