CONCEPT
The Grue Paradox
Goodman's 1955 challenge to induction—'grue' (green before time
t, blue after) is equally well-confirmed by observations as 'green,' yet projects incompatible predictions.
The grue paradox is
Nelson Goodman's most famous contribution to philosophy of science, introduced in
Fact, Fiction, and Forecast (1955). The paradox demonstrates that the logic of inductive reasoning is not as straightforward as the empiricist tradition assumed. Define the predicate 'grue' as follows: an object is grue if it is green before some specified future time
t and blue thereafter. Now consider all observed emeralds, which are green. These observations support two incompatible generalizations: (1) all emeralds are green, and (2) all emeralds are grue. Both generalizations are equally well-confirmed by the evidence, because all observed emeralds are both green and grue (they were observed before time
t). But the generalizations make contradictory predictions: the 'green' hypothesis predicts emeralds examined after time
t will be green; the 'grue' hypothesis predicts they will be blue. The evidence does not distinguish
between the hypotheses, yet rationality requires choosing between them. Goodman's solution: the choice depends on entrenchment—'green' is a projectible predicate because it has been used in successful inductions across centuries