Barker and Achinstein on Goodman

Barker and Achinstein think that it is not possible for a predicate like grue to serve as well as a predicate like green in the role of a qualitative or non-positional predicate. Their arguments consist in a number of attempts to show that one who possesses green in his language can do things with that predicate which one who must work with grue instead cannot do. However, they succeed in showing only that a qualitative predicate is better adapted to our needs than its positional counterpart, rather than that grue is not capable of being a qualitative predicate.