> That blows my mind ... a bit like how do you know the color green is green. Maybe it's purple, but you have been told by someone else that it's green.
You may be interested in the "the map is not the territory" idea. "Green" is not a property of an object but of an observer, though "emits light at wavelength N" is a property of an object.
Though I don't understand why it's in principle impossible for Mary to deduce what colour feels like to a standard human. Just because humans are not smart enough to model a being's internal state completely, given just the total physical information, doesn't mean it's impossible.
"Green" is not a property of an object but of an observer,
But "Green" happens to be a property of other large families of objects -- especially animate objects (foliage, certain insects, birds, and fish) and, more rarely, certain inanimate but nonetheless "special" objects in nature (features in geology; the sky at certain times; and of course, rainbows).
So in that sense -- while "Green" by itself doesn't seem to have intrinsic properties besides an associate to a certain band of the electromagnetic spectrum -- it does have a strong (extrinsic) association to objects which do have interesting intrinsic properties.
I don't really understand your point. "Green" is a label we apply to things which fall into a certain category: namely those which under ordinary circumstances emit or reflect light of a certain wavelength. The observer-dependent things here are:
- the definition of the category (which varies depending on what "ordinary circumstances" are for the observer: people draw the boundaries of light-wavelengths differently), and
- making the judgement "this object is in/is absent from the Green category" for any given object (since our information is imperfect, and [for instance] we may only ever see an object while it is bathed in blue light).
My post was mainly intended to say "there's no paradox involved if you experience green objects differently to me: the word 'green' corresponds to a quale which isn't an inherent property of things in the universe, but an artefact of our experience". Additionally, in this comment, I point out that 'green' can indicate different qualia to different people anyway.
You may be interested in the "the map is not the territory" idea. "Green" is not a property of an object but of an observer, though "emits light at wavelength N" is a property of an object.