We see a stone which is green. Then we see a stone of the same kind which is green. Then we see ten stones of the same kind which are all green as well. We now think that the next stone of this kind is green, too. Why do we think that? We have seen a lot of stones of this kind and found all of them to be green.
Here is where things get weird. All of the stones we have seen are green, but they are all grue as well. ‘Grue’ means green and seen or blue and not yet seen. Since all the stones we’ve seen are green and seen, they are all grue. But here’s what’s weird: we do not think that the next stone of this kind is grue. As the next stone of this kind is not yet seen, it is grue just in case it is blue, and we do not think that it is blue. We think that the next stone is green, and it can’t be green and blue at the same time.
But why do we think this way? What makes us so sure that it’s good to think that the next stone is green and not so good to think that it’s grue?
It might be that there are no grounds for us to think this. It’s just that we are quite used to green and not so used to grue. But if we meet some strange folks who are used to grue and not so used to green, we can’t tell them that they got it wrong. These folks might think that the next stone is grue. That’s just the way they roll, and that’s the long and short of it. We don’t roll that way, but who is to say which way to roll is right? Once we see the next stone, at most one of us can be right, but how can you know who is right when you have not yet seen the next stone?
Or it might be that ‘green’ picks out a more ‘real kind’ then ‘grue’. You might think that two green things are ‘the same’ in a way in which two grue things are not—one grue thing might be green and the next grue thing might be blue, and these are not the same. In this case, it might be that we do have grounds for our thought that the next stone is green and not grue. But the hard part is to work out what it means to say that green things are ‘the same’ in a way grue things are not. True, the green things are all green, but the grue things are all grue, and does that not mean that all the grue things are ‘the same’ in some kind of way?
Last but not least, it might be that green is no more of a ‘real kind’ than grue, but we still have grounds for our thought that the next stone is green and not grue. Then we should try to find out what these grounds are. And we should make sure that the strange folks who like grue more than green can’t use the same grounds for their thought that the next stone is grue and not green. It would be bad if they could just steal our work to prove that they are right and we are wrong.