Grue

Grue is a term in the philosophy of language and epistemology used to illustrate a problem of induction. It was introduced by Nelson Goodman in Fact, Fiction, and Forecast (1954). A grue is a predicate that applies to an object only if the object is green before a designated time t and blue after that time. The analogous predicate with the colors swapped is bleen (blue before t and green after t).

The familiar example involves emeralds. Up to time t, all observed emeralds are green. If they are grue, then after time t they should be blue. Based on observations prior to t, we could just as well predict that future emeralds will be green and that future emeralds will be grue, leading to a conflict in which simple induction seems to fail to distinguish between alternative, equally consistent generalizations.

Goodman’s point is not that color perception is illusory, but that which predicates count as legitimate, projectible generalizations depends on more than raw observations. The problem challenges the assumption that future instances will resemble past ones in any straightforward way and highlights the role of background knowledge, simplicity, and the selection of natural predicates in guiding induction.

Grue has become a standard example in discussions of confirmation theory, scientific reasoning, and the philosophy of science. Critics have offered various responses, typically invoking notions of natural kinds, theoretical virtues, or probabilistic/rational-choice frameworks to explain why some predicates (like green) are preferred for projection over others (like grue).