What's a grue?
I came across the term 'grue' when learning about the new riddle of induction by Nelson Goodman. When forming an inductive inference Nelson argues that all inductive arguments form sentences with law-like predicates. The general idea is, 'all observed A's are B's, therefore all A's are B's'. An example would be, 'all observed ice cubes are cold, therefore all ice cubes are cold'.
Some generalized predicates produce law-like statements, others do not. An example a sentence with a predicate that does not produce law-like predicates is, 'all observed stars are visible, therefore all stars are visible.'
The new problem of induction as Goodman states is that there is no way to distinguish the statements with law-like predicates with sentences with non-law-like predicates. Take the statement, 'all observed emeralds are green, therefore all emeralds are green'. This is where Goodman introduces the word 'grue'. A grue is the color green up until time T then ever after it is blue. So the argument, all observed emeralds are green, therefore all emeralds are green' is just as valid as, 'all observed emeralds are grue, therefore all emeralds are grue'. The problem is that the latter sentence's predicate is not law-like, because 'grue' in the future means blue. Using the word 'green' and using the word 'grue' are both equally valid predicates. The only reason why we do not use the word 'grue', according to Goodman, is because we are not used to the word.
My question is not concerning the validity of the new problem of induction (which I may have butchered in my rewriting). My problem is with the coherency of the predicate 'grue'. I've never heard of a predicate that denotes two different kinds of properties at different times. The notion seems odd. Perhaps someone can help me out...
Some generalized predicates produce law-like statements, others do not. An example a sentence with a predicate that does not produce law-like predicates is, 'all observed stars are visible, therefore all stars are visible.'
The new problem of induction as Goodman states is that there is no way to distinguish the statements with law-like predicates with sentences with non-law-like predicates. Take the statement, 'all observed emeralds are green, therefore all emeralds are green'. This is where Goodman introduces the word 'grue'. A grue is the color green up until time T then ever after it is blue. So the argument, all observed emeralds are green, therefore all emeralds are green' is just as valid as, 'all observed emeralds are grue, therefore all emeralds are grue'. The problem is that the latter sentence's predicate is not law-like, because 'grue' in the future means blue. Using the word 'green' and using the word 'grue' are both equally valid predicates. The only reason why we do not use the word 'grue', according to Goodman, is because we are not used to the word.
My question is not concerning the validity of the new problem of induction (which I may have butchered in my rewriting). My problem is with the coherency of the predicate 'grue'. I've never heard of a predicate that denotes two different kinds of properties at different times. The notion seems odd. Perhaps someone can help me out...
Comments (10)
Allow me to present "mortal".
On the face of it theres nothing absurd about a predicate that would suggest different observables properties depending on time. There's plenty of examples, aren't there? But it seems, from the OP, that 'grue' is supposed to be doing something much more specific.
It's not very new, though. The New Riddle of Induction is the fourth chapter in Goodman's book Fact, Fiction and Forecast, first published in 1955 and adapted from lectures given in 1953. It is related to Kripke's 'quus' alternative rule for addition discussed in his Wittgenstein on Rules and Private Language (1982).
We do have mood rings now which change between green and blue (and other colors too) depending on time.
Really for any entity which changes predicates with time we could invent some predicate which functions like grue. It may be an odd notion, but so what? The force of habit could overcome that odd feeling. Perhaps if we were not very particular about which of the two predicates happen to hold right now we'd invent some third term that's less precise but more efficient.
Or perhaps compare the colour 'denim'. A very dark indigo that fades to light blue. Anyway, objecting to the concept seems a weak argument.
I thought Goodman proposed a predicate that involves a scheduled meaning-change of a word, rather than word that describes a change in an object. Am I wrong?
The implicit time signature is not there in statements that explicitly quantify the time variable, such as 'Neville Chamberlain visited Munich in 1939' or 'Beethoven never married'.
If we apply the same discipline to uses of the word 'grue', of insisting on an explicit time signature as part of the proposition, wherever that makes sense, I think the anomalies that 'grue' is supposed to throw up disappear.
Like my age? That changes on my birthday? I am always "my age," but my age changes. "people my age remember the assassination of President Kennedy." A stable truth using a changing predicate.
Like I said before, trying to rule out 'grue' on grounds of language rules is weak. It is missing the point.
Hm, this is actually surprisingly difficult to answer for me.
On the surface of it, I have an easy "no, not like that". "My age" has a stable meaning, no matter when you say it. "Grue", in my reading, does not. "Grue" does not mean "first green, then blue". It means "either green or blue, depending on which side of time T we check".
There is something they have in common though: they both invoke context. To endow "my age" with meaning, you need to know who speaks and - approximately - how old s/he is. To endow "grue" with meaning, you need to know when the utterance is spoken in relation to time T.
At the same time, though, there's still a difference. You can point at a picture of a man and say "that's a man my age", and if it was true when the picture was taken, it's still true when you look at the picture. However, if you take a picture of grue object and look at it after time T, you're not looking a grue object, even though the object was grue when you took the picture and the colour hasn't changed.
Similarly, when you say "I want to see a grue thing," you know that you want to see either a green or a blue object, and that seeing a green thing too late or a blue thing to early won't count.
Words like "my" are indexical. Words like "mortal" describe a typical form of change. Words like "grue"... have something much like natural language change worked into the definition? The closest real-life equivalent I can think of is applying legal terms when laws change the interpretation of the terms at a certain date (except it's defined into a word from the get go and isn't actually change; you could define "grue" as undergoing a the meaning change every other day (even/odd dates), except defining it like this creates a regularity you can observe and isn't very useful for challenging induction).