I've never heard of a predicate that denotes two different kinds of properties at different times. — Purple Pond

Allow me to present "mortal".

I'm not familiar with Goodman's new problem, but I get the sense that 'grue' is an ad hoc term that has been tailor-made to throw a spanner in the work of some very precise formal machinery. I mention this not to slam 'grue' - in philosophy, ad hoc term (concepts, situations etc) are all important. I mention it because I have the feeling that there is a boatload of preliminary argumentation leading up to its introduction, and its hard to tell, based on the OP alone, the precise work its supposed to do.

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.

I'm not familiar with Goodman's new problem — csalisbury

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).

Everyone knows that you need to carry a torch into the dungeon to fight grues.

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.

Allow me to present "mortal". — unenlightened

You mean 'mortal' is a predicate that denotes being alive at one point time, and dead at another point of time?

Yes, it was what came to mind immediately. But to be mobile implies being in one place at one time and another place at another time, as does to be running.

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?

I think the confusion arises from the fact that most propositions implicitly implicit contain the words 'at <insert time proposition is stated>.' So the statement 'Bill is a bachelor', made in 2017 is actually 'Bill is a bachelor in 2017' whereas, when stated in 2020 it is 'Bill is a bachelor in 2020'.

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.

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? — Dawnstorm

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.

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. — unenlightened

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).