exception proves the rule, the. A widely misunderstood expression. As a moment's thought should confirm, it isn't possible for an exception to confirm a rule - but then that isn't the sense that was originally intended. Prove here is a 'fossil' - that is, a word or phrase that is now meaningless except within the confines of certain sayings ('hem and haw', 'rank and file' and 'to and fro' are other fossil expressions). Originally prove meant 'test' (it comes from the Latin probo, 'I test'), so the exception proves the rule meant - and really still ought to mean - that the exception tests the rule. The original meaning of prove is preserved more clearly in two other expressions: 'proving ground' and 'the proof of the pudding is in the eating'.