Idiom Corner: Does the Exception Prove the Rule?

‘The exception that proves the rule’ is one of the most misused phrases in English. Yet it’s also one that most people will have heard, so it’s time to clarify how it should be used.

So strap in, sit back and enjoy this guide to when (and how) an exception can prove a rule.

The Origins of ‘The Exception that Proves the Rule’

This phrase has its origins in an old Latin legal principle, which stated that ‘the exception confirms the rule in cases not excepted’. This means that when there is an exception to a rule, it shows that there must be a rule there (even when it isn’t explicitly spelled out).

For example, if you see a sign saying ‘No food or drink in the library’, you can guess that food and drink is allowed elsewhere. 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’).

So for any true pedants out there, this is the correct way that an exception can prove a rule. But this is not how the phrase is commonly used these days.

Its Modern Usage

Since old Latin legal principles aren’t that popular any more, this phrase has taken on a new meaning: i.e. ‘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 ‘proofreading’. In all of these cases, ‘proof’ is a case of testing something to check that it’s valid or correct.

An exception ‘proves’ a rule if it makes us question it or reject it. For example, we might believe the rule that ‘everyone loves pizza’. The existence of one person who does not like pizza would then ‘prove’ or ‘test’ the rule.

You’ll want to avoid this usage in formal writing, as it is based on a confusion. But in conversation, people will know what you mean if you use the phrase this way.

How Not to Use the Phrase

There is, however, another way this phrase is used that makes no sense. This is when ‘the exception that proves the rule’ is taken to mean that it can confirm a rule.

With this usage, finding someone who doesn’t like pizza would be taken as evidence that everyone loves pizza. This is clearly absurd, as well as unfair on non-pizza fans (especially if you force them to eat one).

So take note: never use ‘the exception that proves the rule’ to mean ‘the exception confirms the rule’ if you want to be understood. If you think about it, it’s just silly.

Summary: The Exception That Proves the Rule

To summarise, this phrase has two common uses:

1. In formal writing, an exception can ‘prove’ the existence of an unstated rule (i.e. if there is an exception, there must be a rule to which it is an exception). This was the original meaning of the phrase, but it is rare in modern English outside of fairly formal or academic writing.
2. 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.