Grue (color)
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Grue is an artificial adjective, coined from "green" and "blue" by philosopher Nelson Goodman in one of the seminal works in the philosophy of science, Fact, Fiction, and Forecast. The word is used to illustrate what Goodman calls the new riddle of induction.
The word is defined relative to an arbitrary time t as follows: An object satisfies the proposition "x is grue" if and only if it is examined before t and is green, or is examined after t and is blue.
The problem is this. Assuming t has yet to pass, then every emerald that has been observed is both green and grue. But surely we believe that emeralds discovered after t will be green, not grue. So it seems that a green and grue emerald discovered before t is evidence only for "All emeralds are green", not evidence for "All emeralds are grue." The problem is to explain why.
The most obvious response is to point to the artificially disjunctive definition of grue. But this move will not work. For if we take grue and bleen as primitive, we can define green as follows: An object satisfies the proposition "x is green" if and only if it is examined before t and is grue, or is not examined before t and is bleen. To deny the acceptability of this disjunctive definition of green would be to beg the question.
Another possible resolution of the paradox 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. This may beg the question of 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.
Casually, "grue" is often used to mean "green before some time T, and blue on or after some time T, where T is in the future" (Before Y2K, January 1st, 2000 was frequently used for T) , which slightly simplifies Goodman's definitions.
It should be noted that the Goodman's definition does not require that any objects change colour; grue is, strictly speaking, not a colour but a complex property that is a function of both colour and time of first observation. However, the casual definition has often been used to generate a different, epistemological problem: suppose that to be grue is, instead, to change color, from green to blue, at time t. Assuming time t is in the future, how do you know whether or not emeralds are grue? There is no evidence prior to time t capable of deciding the matter.
Example
A real-world example of the concept of bleen and grue, is what could be called "turkey induction": a farmyard turkey could use induction to conclude that the farmer's wife is a supplier of food, although of course she will become executioner. More formally a property is defines as "giving me food before Thanksgiving and slaughtering me on Thanksgiving."
Grue as used in Distinguishing "blue" from "green" in a language
"Grue" has also been used as a blanket term to translate colour words in some languages. A large number of the world's languages, including Welsh and Ubykh, do not distinguish colour terms for "green" and "blue", using the same word for both. The word "grue" is occasionally used to translate these colour terms.
See also
- Bleen
- Grue (monster) from the adventure game Zork
- Distinguishing "blue" from "green" in languagefr:Paradoxe de Goodman