Free Labyrinths of Reason by William Poundstone Page A

“Grue” can be defined using “green” and “blue,” or “green” can be defined using “grue” and “bleen.” Asking which is the more fundamental term is like asking whether the chicken or the egg came first.
    To get the full impact of this, imagine that “grue” and “bleen” are not made-up terms of a logic paradox but the very real terms of a natural language. Native speakers routinely say grass is grue and the sky is bleen. To them, saying that a dress is bleen does not raise the question of how, physically, it is going to turn blue at the turn of the century. (Any more than our saying that a banana is yellow implies that it will never turn brown but will be yellow forever.) When they say a dress is bleen it is because it is being perceived as bleen
right now
. It is the same color as the portion of a color chart that is labeled “bleen”; the color of the bleen sky or the first bleenbird of spring. The only difference between their bleen and our blue is that this time clause is built into the definition (or is it?).
Counterfactuals
    The grue-bleen paradox is partly about
counterfactuals:
terms that talk about what
would
happen even though it
hasn’t
. A paper clip is flexible, acid-soluble, and meltable. It is all these things even if it is never bent, dissolved in acid, or melted. A grue emerald would be grue even if it is destroyed before 1999.
    Counterfactuals abound in science. Following Goodman, astronomers might call the color of the sun “yelite.” The sun is an average yellow star now, and will be a white dwarf in about 10 billion years. Of course, no one has ever observed the sun change from a yellow star to a white dwarf. No one has observed any star do that. All our direct experience confirms that the sun will be yellow forever as well as that the sun is “yelite.”
    What is the difference between this and Goodman’s paradox? The astronomers’ belief in the future change is not accidental. It is not the result of there happening to be a term “yelite” in somebody’s dictionary. It is based on astrophysical theory that has been confirmed in other realms.
    Terms like “grue” and “bleen” are suspect because they arbitrarily delay refutation until a future time. No possible experimentconducted in the twentieth century can distinguish a grue emerald from a green one. The future color change is a supposition that is (so far) unnecessary. For that reason we are justifiably suspicious of someone advancing a hypothesis that emeralds are grue.
    True as this is, it does not make the paradox vanish. Again, the infernal symmetry of the situation rears its double head. The Gruebleen-speaking jeweler can complain that no twentieth-century experiment will aid him in deciding whether a grue emerald will turn bleen in the year 2000 (this being his definition of “green”). To resolve the paradox, even partially, you must find a way in which the situation
isn’t
symmetrical.
The Rotating Color Wheel
    Maybe the problem is the
suddenness
of the change. Sudden changes generally require a cause. In a vacuum, an object may continue its motion forever, but an abrupt change of velocity can come about only through an outside agent.
    If the suddenness bothers you, let “grue” describe a gradual change from green to blue over a period of a thousand years. Better yet, suppose that all the colors are changing. The artist’s color wheel is slowly rotating, so to speak, and what is green now will be blue in a thousand years, purple in 2000 years, red in 3000 years, and come full cycle back to green in 6000 years. “Grue” applies to that class of objects (emeralds, summer leaves, etc.) that are green now, blue in a thousand years, and so on.
    Assuming a 6000-year cycle, the color of all things would change ever so slightly with each passing moment. The cumulative color change in a human lifespan would, however, be so minor that hardly anyone would notice. (The wizened gemologist who complained