At first glance, “the proof is in the pudding” seems thoroughly mysterious. What proof, in what pudding? Does this have anything to do with Colonel Mustard in the study with a candlestick? But the key to the mystery lies in the fact that “the proof is in the pudding” is actually a mangled form of the original phrase, which was “the proof of the pudding is in the eating.” A dish may have been made from a good recipe with fresh ingredients and look delicious, but you can really only judge it by putting it in your mouth. The actual taste is the only true criterion of success.
“The proof of the pudding is in the eating” is a very old phrase, dating back to at least 1605, and “proof” in the adage is an antiquated use of the word in the sense of “test” (also found in “printer’s proof,” a preliminary “test” copy of a book printed to check for errors, etc., before commencing a large print run).
Just how and why “the proof of the pudding is in the eating” was shortened to the semi-nonsensical “the proof is in the pudding” remains a mystery, but it’s worth noting that most people now interpret “proof” in the sense of “conclusive evidence.” That’s probably just as well, since “the test is in the pudding” would make, if possible, even less sense. In any case, “the proof is in the pudding” is hardly the only English idiom that doesn’t make any sense if read literally, and it certainly serves a useful purpose, even if it does sound like a cryptic clue from a Sherlock Holmes story.