The exception that proves the rule.

Why do the family always look to me to explain things? "What does this saying mean?" ask the grandchildren. "Ask your grandad" is the answer! Well it's true that "prove" here is being used in its original meaning of "to test; to put to the test", but I don't think that takes us much further. After all, if you test something, then you show it to be so, or not so - that is, you prove or disprove it - so the two meanings are pretty much the same. No, I think the word which causes more of a problem is "rule". Today we tend to associate that word with law, regulation, things laid down by those in authority which must be complied with, whereas originally it meant no more than "what normally is" - the meaning we still use when we say "...as a rule...". Now if you bear that definition in mind, it becomes clear that the basic meaning of the expression is that if you have an exception, then there must be a rule for it to be an exception to - the very existence of an exception proves the existence of the rule. Suppose for example, that you drive into a town where you've never been before, and notice signs everywhere saying "Free parking today". What do you understand from this - well, clearly, that you can park for free today, but also that normally (that is to say that other than today) you would have to pay for the privilege. So the exception (you can park free today) proves [the existence of] the rule (normally you have to pay to park here). OK?