AIBU to not understand how the exception proves the rule?

[J2Od]

Please help me understand how the exception is supposed to prove the rule.

That there is a deviation from the rule proves the rule exists.

I.e, "a black swan" shows us that the general rule is that swans are white. A sign that says "No parking on Sundays" shows the rule that parking is mostly allowed (on all other days).

In other words, the reason the exception stands out and you feel the need to point it out is proof the rule exists in general.

I think it means, for example, if you let one person wear trainers (against the rules) then more people will do it and then the whole school looks less smart (proves why they had this rule)

If ypu provide an exception then the implication is that a rule does exist, so if a sign says ‘no parking during 7am and 9am’ then the implication is that the rule is you can park there at all other times. ‘Guidedogs only’ implies a rule that no other animals allowed and so on.

That it's an exception proves that there's a rule.

@Agixparking example, and @InfoSecInTheCityare right.
But people often misuse this saying - I’d say the black swan example is an example of it being misused. The existence of a black swan is not “the exception that proves the rule”. It actually disproves the “rule” that swans are white. Saying “free parking on weekends” does show the existence of the rule that parking is not free during the week, and is therefore the exception that proves the rule.

In this adage, “prove” means “test” not “offers evidence.”

So “the exception tests the rule.”

The exception isn't 'when it goes wrong'. It's a 'but in these circumstances the rule doesn't apply'. So the fact that there are circumstances where the rule doesn't apply means that the rule is true in every other cicumstance.

This. It’s “prove” in the now little-used sense of “test”. So, if there is a generally applicable rule, anything which is an exception to it tests whether the rule works/is a good one.

In medical terms it is that there exists a normal that has been defined, (the rule) and that deviation from that is the exception - so we have a normal range of blood results and if you are outside this, your results are abnormal.

Without the rule or the normal, there can be no idea of abnormal to see where things are going wrong.

"Models are skinny."

"But there's a plus size one on the cover of this month's fashion magazine!"

"I know without looking it'll be Tess Holliday."

"How did you know that?"

"Because it's always Tess Holliday. There are almost no famous plus size models. She's the exception that proves the rule."

Thanks all. Yes @WhatAMarvelousTune I can see the difference between the 2 examples and agree that the swan one doesn't really work

@ParmaVioletTea and @friedaddedchilli --the different meaning of 'prove' is a helpful take

Well, it’s the historically correct meaning of the phrase.

“Proves” used to mean tests rather than confirms.

So it’s the exception that tests the rule

Well, the original expression means 'the exception TESTS the rule' - ie 'prove' meaning 'test' in the past. So the precise opposite of how we now use the expression!

All Mammals have hair.
A bird does not have hair
A bird is not a mammal

All men have a y chromosome
Women do not have a Y chromosome
Women are not men

Bella has a Y chromosome
Bella is not a woman

It's not a "take", it's what it means.

Very interesting. The meaning has changed over time though and this is not how the expression is used these days.

It's a phrase that's lost its truth because of language change, as (a minority of) PPs have correctly said. Nowadays it's what people say because they don't have any other more compelling argument. I don't think it's worth looking for meaning in it beyond that.

"The proof of the pudding is in the eating" is another phrase where proof mean test, not evidence.

It's like "desert island" - it means deserted, no one living there, not a sandy, arid island.

Or "get their just deserts" which uses desert (pronounced dessert) to mean "that which you deserve".

Lots of sayings use earlier meanings for words.

Well, the original expression means 'the exception TESTS the rule' - ie 'prove' meaning 'test' in the past. So the precise opposite of how we now use the expression.

Exactly - it tests the rule and finds the rule to be not a rule.

Because it’s the exception

'Prove' used to mean test. It comes from the Latin probare, which is also the root of the verb 'to probe'. So the original meaning was 'the exception that tests the rule'.
As language changes, archaic uses often remain in phrases and idioms long after they've fallen out of use (like e.g. 'at your beck and call' (what's a beck?). What's interesting about this phrase is that we've bent the meaning of it to fit the language change. It's original meaning was much more straightforward and logical. I find all this stuff fascinating!