Goodman’s Grue, is about the limitations of induction. Suppose you have two hypotheses, namely “emeralds are green”, and “emeralds are green and will so remain until New Year’s Day 2050, at which point they will all turn blue.” (This can also be expressed by saying that “emeralds are grue”, where “grue” is defined as “green until 2050 and blue thereafter”.) Statistically, both of these hypotheses are equally confirmed by current observations.

— Christopher Michael Langan, The Art of Knowing