Yesterday I heard an English baker on a cooking show say that "the proof is in the pudding." I've heard the expression before but I can't imagine how pudding would prove anything. How did the idiom start, what did it mean originally, and how is it correctly used today?
William Camden, Remains Concerning Britain: Their Language, Names, Surnames, Allusions, Anagramms, &c. &c. &c. (1674) has this formulation
The proof of a pudding is in the eating.
which means simply that a pudding is as good as it tastes—or conversely, that if it doesn't taste good, no amount of justification offered on behalf of its other merits or initial disadvantages really matters.
Oswald Dykes, English Proverbs, with Moral Reflexions (1713) offers this moral reflection on the proverb:
In fine, the best Way of judging of Things beyond Mistake, is by the Issue, or the Event of them ; for as we say commonly, The Proof of the Pudding is in the eating, so we ought to speak as we find only upon the last Tryal and Experience, either of its Goodness, or its Distaste.
Charles H. Spurgeon, The Salt-Cellars (1889) frames the test—and the proverb—as a negative:
The proof of the pudding is not in chewing the bag.
after which he draws a religious analogy:
No ; nor does the proof of soundness in the faith lie in using the phrases of orthodoxy, and harping on mere words.
So if true faith is the pudding, "the phrases of orthodoxy" are the bag.
To prove in this context means to test (cf. Danish 'prøve', German 'probieren', 'to try', 'to sample', 'to try out'). Although it is almost obsolete in normal usage today, this less common application of the verb 'prove' is still current in certain familiar collocations, e.g. 'to prove [= test] the suitability' of someone or something, 'to prove [= test] ore', and in the term 'proving ground'.
The same sense of the word is the one being invoked in it's the exception that proves the rule. (If you substitute test, the proverb actually makes sense: it's the exception that tests the rule.)