Most of us have heard that “the exception proves the rule” doesn’t mean what it might first appear to mean — that an exception to a rule somehow demonstrates the rule’s greater truth — and that “proves” simply means “tests”:
The common misconception (which you will find in several books, including the Dictionary of Misinformation) is that ‘proves’ in this phrase means ‘tests’. That is not the case, although ‘proof’ does mean ‘test’ in such locutions as ‘proving ground’, ‘proofreader’, ‘proof spirit’, and ‘The proof of the pudding is in the eating.’
The phrase actually has a fairly straightforward legal meaning. An example demonstrating the original legal sense:
‘Special leave is given for men to be out of barracks tonight till 11.0 p.m.’; ‘The exception proves the rule’ means that this special leave implies a rule requiring men, except when an exception is made, to be in earlier. The value of this in interpreting statutes is plain.
Or, as Lord Atkin said:
A rule is not proved by exceptions unless the exceptions themselves lead one to infer a rule.
Exceptio probat regulam in casibus non exceptis. The exception proves the rule in cases not excepted.