“The exception that proves the rule” is a commonly misused phrase in English. Yet it’s also one that most people have heard, so let us clarify how an exception can prove a rule.
The Exception That Proves the Rule
This phrase has its origins in a Latin legal principle that stated “the exception confirms the rule in cases not excepted.” In other words, when there’s an exception to a rule, we know that there must be a rule to which it is an exception (even when this rule isn’t explicit).
For example, if you see a sign saying “No food or drink in the library,” you can work out from this alone that food and drink is allowed in other places. So the exception (i.e., “No food or drink in the library”) proves that another rule must exist (i.e., “Food and drink is permitted outside of the library”).
This is the original use of the phrase and still the “correct” use for many passionate pedants. But it is not what most people now mean by “the exception that proves the rule.” Read on to find out more.
Old Latin legal principles are not all that popular anymore. Consequently, the phrase “the exception that proves the rule” has taken on a new meaning. Nowadays, then, it usually means the exception that tests the rule.
This is based on a definition of “proves” that we also see in phrases like “proving ground,”“the proof of the pudding is in the eating,” and even in “proofreading.” In all these cases, “proof” means test something to check that it’s valid or correct. As such, an exception can “prove” a rule if it makes us question it (or even reject it). For example, we might believe “everyone loves pudding” as a rule. But the existence of one person who hates pudding would then be an exception that “proves” or tests this rule.
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You’ll want to avoid this usage in formal writing, as it is based on a confusion. But people will know what you mean if you use “the exception that proves the rule” this way in conversation.
How Not to Use the Phrase
This phrase is used in another way sometimes: i.e., taking “the exception that proves the rule” to mean an exception can confirm a rule.
Unfortunately, this does not make sense because it involves a direct contradiction. For instance, let us return to the world in which “everyone loves pudding” is a rule. If an exception could “confirm” this, we would have to treat someone who hates pudding as “proof” our original rule was true. And this is clearly absurd, as well as unfair on people who don’t like pudding.
Summary: The Exception That Proves the Rule
To summarize, this phrase has two common uses:
In formal writing, an exception can “prove” the existence of an unstated rule (i.e., if there is an exception to a rule, there must be a rule to which it is an exception). This original use of the phrase is rare in modern English.
The modern use of this phrase is to mean “the exception that tests the rule” (i.e., an exception that makes us question a rule).
However, you should never use this phrase to mean “the exception confirms the rule.” This would be incorrect and illogical. And if you want someone to make sure you’re using idioms correctly, let us know.