Idiom Corner: The Exception that Proves the Rule

Welcome back to Idiom Corner, where we look at confusing figures of speech. Today, we have a particularly baffling one: ‘The exception that proves the rule.’

There are at least three ways this phrase is commonly interpreted. Two make some kind of sense; one really doesn’t. In this post, then, we unpack how ‘the exception that proves the rule’ should be used.

How Not to Use ‘The Exception that Proves the Rule’

We’ll get the nonsensical interpretation out of the way first. This takes the phrase to mean that something can contradict a rule yet also confirm it.

‘Prove’ is understood, in this context, to mean ‘establish the truth of something’. If we interpret it this way, we can use the phrase to dismiss problematic counterexamples to a trend or rule.

However, this common interpretation becomes a little problematic if you think about what it implies. It would be like saying that ‘every swan is white’ and then taking the existence of a black swan to confirm this statement.

A Different Type of ‘Proof’

A second common interpretation is to take ‘the exception that proves the rule’ as meaning ‘the exception that tests the rule’.

This takes ‘proves’ to mean ‘to test the validity of’. This sense of ‘prove’ is used in phrases like ‘proving ground’ and ‘the proof of the pudding is in the eating’. The ‘proofs’ that proofreaders read are related to this too, since they are ‘test’ prints. In this example, finding a black swan would be the exception that makes us question the rule that ‘every swan is white’. But while this use of the phrase is broadly accepted, its original meaning was a little different.

The Original Meaning

We have to go a long way back for this one, as ‘the exception that proves the rule’ is actually adapted from a legal principle developed in Republican Rome: ‘The exception confirms the rule in cases not excepted.’

For example, a sign saying ‘No parking between 9AM and 6PM’ would imply that parking is allowed at other times. Hence, the exception (i.e. the period when parking is forbidden) proves the rule (i.e. parking is otherwise permitted).

If you want to be truly pedantic, then, only use ‘the exception that proves the rule’ when an exception implies a rule in cases unlike the exception (rather than proving the truth of a rule via a counterexample).

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. that if an exception exists, 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.