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What does it mean to prove a rule?

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I recently wrote that Martha Karua’s contribution to a parliamentary debate was “exceptionally inspiring”.

That means that it stood above all others. In common parlance, it was “the exception that proved the rule”. But isn’t it a curious statement?

The instances of a rule are what might “prove” the rule. However, even here, the idea of “proving” a rule is hardly logical.

For what on earth does it mean to “prove a rule”? Most English users – even in England – assume that the verb “to prove” in this idiom means to impose an affirmation.

But, even if it did, even if we needed to “affirm” the rule, how would the exception do it? Where is the logic?

The answer is that, in this idiom, the verb “to prove” has an archaic meaning. It once meant merely “to put to the test”.


By standing in contrast to the instances, the exception tests the rule or, in the jeweller’s term, “assays” it.

In a delightful sonnet, Shakespeare describes things which, like a member of the opposite sex, are often “…past reason hunted (but), no sooner had, past reason hated…”

Such a thing, he writes, is “…a bliss in proof and, proved, a very woe…” While you were “proving” (chasing, testing) it, it looked heavenly.

But, upon proof (on attaining it), it turned out to be more hideous than Echidne’s brood. Do not ask me if the pudding is delicious. Prove it (test it) with your own taste buds.

The verb prove comes from the French prover and the Latin probare (to test). Probare has also given us the verb to probe. This is in line with our theme.

To probe is not necessarily to affirm but merely to “test”, for instance, somebody’s probity.

Probity (integrity) comes from the Latin adjective probus (honest) and has given us the noun probation (a system in which an offender is placed under the supervision of an officer.

The trial period — the time taken to test if the offender has genuinely reformed — is also called probation.

In law, probate is the process of officially testing the validity of somebody’s will. In a back-formation, we have the noun probe, such as the one facing Samuel Kivuitu and his team.

A space probe is a ship or satellite equipped to obtain scientific information — transmitted back to earth by radio — in outer space.

Space probes will one day affirm whether or not our planet is unique — if it is the only one with a technological ability to conduct such probes.

It would make us the exception which proves the rule that the universe moves unconsciously from the energy released by the Big Bang nearly five billion years ago.

The verb to except (to omit or exclude), the preposition except (other than, apart from), the conjunction excepting (unless), the adjective exceptional (extraordinary) and exceptionable (objectionable) and the verbal phrase to take exception (to object) come from the Old French excepter (to leave out) and the Latin excipere (to take out).

The author is a veteran journalist