Revealed preference theory (Samuelson, 1938): you don't need to know what people say they prefer — observe what they choose, and you've revealed their preference.
The insight was a methodological breakthrough: economics could be freed from introspection and psychology. Choices are observable; mental states aren't.
The problem: revealed preference can't distinguish between a choice that reflects stable preferences and one that reflects a mistake, a misunderstanding, or a choice under constraint. The theory treats all choices as preference-revealing, which is too permissive.
See: ↗ @dave / bayes-as-updating — updating beliefs in light of choices is structurally similar.