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It's grue, not green in the predicate, that's the whole point. Probably a long-lived typo |
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The problem is this. Let ''t'' be some moment in the future. Then "All emeralds are green" and "All emeralds are grue" are ''both'' true. So long as ''t'' has not yet arrived, every green emerald we find agrees with both sentences, but surely a green emerald is evidence only for "All emeralds are green", not evidence for "All emeralds are grue." The problem is to explain why not.
Casually, "grue" is used to mean "[[green]] before [[January 1]], [[2000]] and [[blue]] on or after [[January 1]], [[2000]]"; sometimes this is useful for metaphysical or epistemological discussions. It should be noted that the above definition does ''not'' require that any objects change colour; grue is,
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