The true value or quality of something can only be judged by putting it to the test or seeing it in action. Results matter more than promises or appearances.
This phrase is actually a shortened version of the 17th-century saying 'the proof of the pudding is in the eating.' 'Proof' here means 'test,' not evidence. The original made clear that you can't judge a pudding's quality by looking at it - you must taste it. The shortened modern version emerged in the 20th century.
Most people don't realize this phrase has been accidentally shortened, which actually makes it less clear! The original 'proof of the pudding is in the eating' perfectly captured the idea that testing reveals truth. Medieval puddings were also quite different - more like sausages cooked in animal intestines, making the testing aspect even more crucial.
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