When enough people know a saying, but don’t understand its intent, its meaning drifts. The idea is supposed to be that the existence of an exception proves that there is a rule, with “rule” being in the sense of a generality.
I learned that from the Grammar Girl podcast and found it interesting enough that it stuck with me (because it was something I’d long wondered). Her example is a sign that forbids skateboarding while school is in session. “You can infer that you are allowed to skateboard at other times. The rule that the exception proves is that skateboarding is generally allowed. If that were not a rule, why would exceptions be made at all?"
When enough people know a saying, but don’t understand its intent, its meaning drifts. The idea is supposed to be that the existence of an exception proves that there is a rule, with “rule” being in the sense of a generality.
I learned that from the Grammar Girl podcast and found it interesting enough that it stuck with me (because it was something I’d long wondered). Her example is a sign that forbids skateboarding while school is in session. “You can infer that you are allowed to skateboard at other times. The rule that the exception proves is that skateboarding is generally allowed. If that were not a rule, why would exceptions be made at all?"
Transcript: https://www.quickanddirtytips.com/qdtarchive/why-do-we-say-the-exception-that-proves-the-rule/
That does make sense, even if it is a completely ass way of phrasing it