I spent the day with a roller, a stepladder, and four chatty females as we painted up a storm. At one point, the conversation in the other room (where it always was) went like this:
Voice One: It's blue.
Voice Two: No, it's green. The label says "green."
Voice One: Look, it's blue. Light blue. Bluish.
Voice Two: Maybe it starts out green, but turns bluer as it dries.
Voice One: Definitely blue.
Grue and bleen, in philosophical parlance, are
colors that create a paradox of induction.
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....
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.
Confusing enough? Don't worry, you're not a professional philosopher, so you can keep on enjoying grue grass and bleen skies.
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