Taking Exception

The current issue of the Economist contains an aphorism that we haven’t seen in a while--which is just as well, since it is always used not only incorrectly but also nonsensically. The saying is, The exception proves the rule.

This piece of wisdom is generally taken to mean that an exception demonstrates or establishes a rule, which is absurd. It implies that all rules have exceptions, and that finding an exception is what makes a rule valid. But exceptions should decrease, not increase, confidence in a rule. What’s the point of having a rule if it doesn’t apply?

The fact is that the word prove has another, earlier, meaning besides demonstrate . It means to test , a meaning that is preserved in the phrase proving ground , which is a place where equipment or ideas are tested, and in the sentence The proof of the pudding is in the eating , which means that using something is the best way to test it. (The printer’s term proof also embodies this meaning. A proof is a test of a piece of type.)

So the sentence The exception proves the rule does not mean that an exception establishes a rule, but rather that an exception tests a rule, which makes perfect sense. Far from demonstrating the rule, finding an exception puts it to the test. Can the rule stand up in spite of the exception, or does it have to be revised?

An exception is not the end of the inquiry. It’s the beginning.