The other exception that proves the rule

In August, I wrote about how I learned the “true” meaning of the expression “the exception that proves the rule”, namely from an essay about mite incest by paleontologist Stephen Jay Gould. In that flavour of the expression, “prove” comes from the Latin probare, meaning to test. An exception can test a rule, for example, by identifying a case where the assumptions underlying the rule no longer hold. In Gould’s case, the rule that sex ratios in animals tend to be equal, except under certain conditions where the reasons for this pattern do not hold.

But there is another flavour of this expression that is more widely cited, derived from a legal defence by the Roman statesman and lawyer Cicero: exceptio probat regulam in casibus non exceptis (the exception proves the rule in cases not excepted). In other words, identifying an exception implies the existence of a general rule. For example, a sign reading “no parking on Sundays” implies that parking is permitted on other days, even if not directly stated.

The most common use remains dismissing a pertinent counterpoint to one’s argument as “the exception that proves the rule”.