Exception that proves the rule

One of the reasons that I wanted to start this blog was to have a place where I could document phrases that are used to mean one thing but mean a completely different thing when you learn more context about them, because there seems to be quite a few of those phrases in English.

Most recently, last month I skimmed a thread on Hacker News 🔗 discussing an article about physics and mathematics, but for me the most interesting comment in that thread was this digression 🔗:

Interesting linguistic aside: “the exception that proves the rule” makes use of this more old fashioned use of prove, to test. So it’s the exceptional case that tests the rule.

…and thereby proves the rule false.

The Wikipedia page for the phrase 🔗 sites 1918’s The Style Book of the Detroit News 🔗:

The exception proves the rule is a phrase that arises from ignorance, though common to good writers. The original word was preuves, which did not mean proves but tests.

The claim is roughly corroborated by Etymonline’s page about “prove” 🔗:

c. 1200, prēven, pruven, proven “to try by experience or by a test or standard; evaluate; demonstrate in practice,” from Old French prover, pruver “show; convince; put to the test” (11c., Modern French prouver)

So if your rule has an exception that proves the rule, then you do not have a rule. Unless your exception turns out not to actually be an exception after all, but that would be the exception that proves the rule.