The exception that proves the rule .... Replying to an old thread that is locked.


The most common meaning of "the exception that proves the rule" is that by stating an exception, a rule is implied. A sign that says "parking prohibited on Sundays" is an exception that implies (proves) the rule that parking is allowed on the other days of the week.
"The exception proves the rule" is a saying whose meaning has been interpreted or misinterpreted in various ways. Its true definition, or at least original meaning, is that the presence of an exception applying to a specific case establishes ("proves") that a general rule exists. For example, a sign that says "parking prohibited on Sundays" (the exception) "proves" that parking is allowed on the other six days of the week (the rule). A more explicit phrasing might be "the exception that proves the existence of the rule."​

However there is also a scientific sense which is what I was using in my post that was quoted.

Scientific sense​
A case may appear at first sight to be an exception to the rule. However, when the situation is examined more closely, it is observed that the rule does not apply to this case, and thus the rule is shown to be valid after all.
Fowler's example is of a critic, Jones, who never writes a favourable review. So it is surprising when he writes a favourable review of a novel by an unknown author. Then it is discovered that the novel is his own, written under a pseudonym. Obviously the rule doesn't apply to this case (although the rule may need to be more precisely stated in future) and the previous evaluation of Jones's ill-nature toward others is re-affirmed.​
An example of this use in science writing is laid out by Richard Dawkins in The Ancestor's Tale. Cnidaria is a phylum of animals including jellyfish, corals, and sea anemones. The rule is that all cnidarians, and only cnidarians, have specialized harpoon cells called cnidocytes, which they often use to capture and/or inject venom into prey. There is one exception to this rule. Some species of sea slugs of the nudibranch group have tentacles containing cnidocytes, even though the slugs aren't cnidarians. But it turns out that the slug eats jellyfish and passes the jellyfish's commandeered weapons, intact and still working, into its own tentacles. So examining the only known exception really proved the original rule valid after all.​