Nelson Goodman (1906–1998) supposed that all emeralds before time T, which is the present, are green. But if this is true, then “G” is also true: “Emeralds before time T are green or emeralds after time T are blue.” The reason it is true that emeralds after time T are either green or blue is that the time after time T is the future and we do not know what the future will hold for emeralds—or for anything else.

G defines the predicate “grue” (a term Goodman made up) as a quality of emeralds: All of the emeralds that qualify as “grue” could be blue after time T. Nevertheless, Goodman maintained that we would prefer to call them “blue” after time T. He believed this showed that confirmation cannot be a purely logical or syntactic process, but that it reflects our linguistic preferences, which go beyond what we actually know.