A word that means 'green until 2030, then blue' breaks the logic of prediction

Nelson Goodman defined 'grue': an object counts as grue if observed before some future date and found green, or observed after and found blue. Every emerald ever seen is equally evidence that all emeralds are green, or that they're all grue — the raw data can't tell the two rules apart. Goodman used the puzzle to show induction quietly assumes which properties are the 'natural' ones worth projecting.

— Nelson Goodman, Fact, Fiction, and Forecast — 1955, 'The New Riddle of Induction'