Grue and bleen

Grue and bleen are predicates coined by Nelson Goodman in Fact, Fiction, and Forecast to illustrate the "new riddle of induction". These predicates are unusual because their application to things is time dependent. For Goodman they illustrate the problem of projectable predicates and ultimately, which empirical generalizations are law-like and which are not. Goodman's construction and use of grue and bleen illustrates how philosophers use simple examples in conceptual analysis.

Goodman defined grue relative to an arbitrary but fixed time t as follows: An object is grue if and only if it is observed before t and is green, or else is not so observed and is blue. An object is bleen if and only if it is observed before t and is blue, or else is not so observed and is green.

To understand the problem Goodman posed, it is helpful to imagine some arbitrary future time t, say January 1, 2027. For all green things we observe up to time t, such as emeralds and well-watered grass, both the predicates green and grue apply. Likewise for all blue things we observe up to time t, such as bluebirds or blue flowers, both the predicates blue and bleen apply. On January 2, 2027, however, emeralds and well-watered grass are now bleen and bluebirds or blue flowers are now grue. Clearly, the predicates grue and bleen are not the kinds of predicates we use in everyday life or in science, but the problem is that they apply in just the same way as the predicates green and blue up until some future time t. From our current perspective (i.e., before time t), how can we say which predicates are more projectable into the future: green and blue or grue and bleen?

In this section, Goodman's new riddle of induction is outlined in order to set the context for his introduction of the predicates grue and bleen and thereby illustrate their philosophical importance.

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