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Grue and bleen are artificial predicates, coined as two portmanteaux of "green" and "blue" by philosopher Nelson Goodman in one of the seminal works in the philosophy of science, Fact, Fiction, and Forecast. The words are used to illustrate what Goodman calls "the new riddle of induction".
Grue and bleen defined
The word grue is defined relative to an arbitrary but fixed time t as follows: An object X satisfies the proposition "X is grue" if X is green and was examined before time t, or blue and was not examined before t.
The word bleen has a complementary definition: An object X is bleen if X is blue and was examined before time t, or green and was not examined before t.
Note. When Goodman originally presented his "riddle", he used a concrete time t in his definitions, namely January 1st, 2000, a date that at the time was far in the future but now in the past. For understanding the problem posed by Goodman, it is best to imagine some time t in the future.
The new riddle
The problem is as follows. A standard example of induction is this: All emeralds examined thus far are green. This leads us to conclude (by induction) that also in the future emeralds will be green, and every next green emerald discovered strengthens this belief. Goodman observed that (assuming t has yet to pass) it is equally true that every emerald that has been observed is grue. Why, then, do we not conclude that emeralds will remain grue, and why is the next grue emerald that comes along not considered further evidence in support of that conclusion? The problem is to explain why.
Responses
The most obvious response is to point to the artificially disjunctive definition of grue. But, said Goodman, this move will not work. For if we take grue and bleen as primitive, we can define green as "grue before t and bleen afterwards", 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 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.
Real-world examples
A real-world example of the concept of bleen and grue might be a traffic light that is red now, and might be assumed to always remain red by a hypothetical group of visiting aliens who live at a much faster pace.
Likewise, a turkey may be led to conclude by induction that the farmer's wife is a supplier of food, rather than a "supplier of food before time t, but executioner at t", where t = Thanksgiving.
Grue as used to translate a color name in natural languages
"Grue" has also been used as a blanket term to translate color names in some languages. A large number of the world's languages, including Japanese, have a single color name for a segment of the visible spectrum encompassing colors that in English would be considered green (and not blue), as well as colors considered blue (and not green). The word "grue" is occasionally used to translate or discuss such color terms.
See also
References
- Fact, Fiction, and Forecast. Cambridge, MA: Harvard UP, 1955. 2nd ed. Indianapolis: Bobbs-Merrill, 1965. 3rd. ed. Indianapolis: Bobbs-Merrill, 1973. 4th ed. Cambridge, MA: Harvard UP, 1983.
"Grue (color)"