Responses
The most obvious response is to point to the artificially disjunctive definition of grue. But, Goodman noted, this move will not work. For if we take grue and bleen as primitive, we can define green as "grue if first observed before t and bleen otherwise", and likewise for blue. To deny the acceptability of this disjunctive definition of green would be to beg the question.
Another proposed resolution of the paradox (which Goodman addresses and rejects) is that "x is grue" is not solely a predicate of x, but of x and the time — we can know that an object is green without knowing the current time, but we cannot know that it is grue. If this is the case, we should not expect "x is grue" to remain true when the time changes. However, one might ask why "x is green" is not considered a predicate of the current time — the more common definition of green does not require any mention of the time of observation, but the disjunctive definition given above does. However, this response also begs the question given that the above definition of blue in terms of grue and bleen also refers to time.
Swinburne gets past the objection that green be redefined in terms of grue and bleen by making a distinction based on how we test for the applicability of a predicate in a particular case. He distinguishes between qualitative and locational predicates. Qualitative predicates, like green, can be assessed without knowing the spatial or temporal relation of x to a particular time, place or event. Locational predicates, like grue, cannot be assessed without knowing the spatial or temporal relation of x to a particular time, place or event, in this case whether x is being observed before or after time t. Although green can be given a definition in terms of the locational predicates grue and bleen, this is irrelevant to the fact that green meets the criterion for being a qualitative predicate whereas grue is merely locational. He concludes that if some x's under examination - like emeralds - satisfy both a qualitative and a locational predicate, but projecting these two predicates yields conflicting predictions - namely, whether emeralds examined after time t shall appear blue or green, we should project the qualitative predicate, in this case green.
Read more about this topic: Grue And Bleen
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