An entry on the linguistic tools used to expose the hidden assumptions behind empirical observation. 4 mins read.
To understand the terms grue and bleen, we must step out of our comfortable assumption that colors are fixed, eternal properties of physical matter. Instead, these terms force us to view color through the lens of time.
Let us select an arbitrary future point in time, which we will call T. We define the two predicates as follows:
- Grue applies to any object that is observed prior to time T and is green, or is not observed before T and is blue.
- Bleen applies to any object that is observed prior to time T and is blue, or is not observed before T and is green.
If we live in a world before time T, every emerald we have ever examined is green. Because these green emeralds have been observed before T, they also perfectly satisfy the definition of being grue. If we use standard inductive logic, we have two equally valid predictions for what will happen to an emerald examined after time T: it will remain green, or it will be grue (and therefore appear blue).
The genius of grue and bleen is that they cannot be dismissed as simply 'more complicated' than green and blue. If we grew up speaking a language where grue and bleen were the primary concepts, we would have to define 'green' and 'blue' using complex temporal clauses. Complexity is relative to our linguistic starting point.
The Symmetry Problem
A common attempt to escape this paradox is to claim that "grue" is cheating because it contains a temporal reference point (time T), whereas "green" is simple and timeless.
But Goodman demonstrated that this is a illusion of perspective. Suppose we start with a language that treats "grue" and "bleen" as primary, natural concepts. In this language, an observer looking at the world would define our traditional colors using the same temporal shifts:
Green would be defined as: any object observed before T that is grue, or not observed before T that is bleen.
From this linguistic starting point, it is "green" and "blue" that look absurdly complicated, artificial, and time-dependent. The choice between these conceptual frameworks cannot be resolved by appealing to simplicity or logic alone.
Referenced Works & Texts
- Nelson Goodman, Fact, Fiction, and Forecast, Chapter IV: "Prospects for a Theory of Projection" (1954). The original formulation of the grue/bleen symmetry.
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