Abstract

When faced with a rule that they take to be true, and a recalcitrant example, people are apt to say: “The exception proves the rule”. When pressed on what they mean by this though, things are often less than clear. A common response is to dredge up some once-heard etymology: ‘proves’ here, it is often said, means ‘tests’. But this response—its frequent appearance even in some reference works notwithstanding1—makes no sense of the way in which the expression is used. To insist that the exception proves the rule is to insist that whilst this is an exception, the rule still stands; and furthermore, that, rather than undermining the rule, the exception serves to confirm it. This second claim may seem paradoxical, but it should not, once it is realized that what does the confirming is not the exception itself, but rather the fact that we judge it to be an exception; and that what is confirmed is not the rule itself, but rather the fact that we judge it to be a rule. To treat something as an exception is not to treat it as a counterexample that refutes the existence of the rule. Rather it is to treat it as special, and so to concede the rule from which it is excepted. The point comes clearly in the original (probably 17th Century) Latin form: Exceptio probat (figit2) regulam in casibus non exceptis. Exception (i.e. the act of excepting) proves (establishes) the rule in the cases not excepted. Clearly this form of reasoning cannot apply when the rule that we are considering has the form of a simple universal generalization. Here there can be no exceptions, only counterexamples. So what we need, and what will be developed..