I believe the original meaning is more like if there is an exception, there must be a rule (otherwise it wouldn't _be_ an exception/you wouldn't need an excepti

I believe the original meaning is more like if there is an exception, there must be a rule (otherwise it wouldn't _be_ an exception/you wouldn't need an exception). It's been watered down a lot, and is often used to suggest that any counterexample makes a rule stronger (which is obviously not always the case -- enough counterexamples just proves the rule is wrong/doesn't exist).