Meaning
I will argue that meaning does not exist.I think it is merely an Emergent property(Yes,I have a thing for Emergent systems).The only catch is that once it "emerged",we had it frozen and caged in our dictionaries,our legal codes,our zeitgeists.
Meaning emerges because of the fact that we,members of the society,have in common a human brain(and a human body,if you are not very Cartesian).Consequently,most of the mental representations excited in us,are more or less the same.More or less enough ,so we dont mind if some authority like a dictionary defines the "meaning" of every word(telling us that these are the descriptions of the mental representations that rightfully deserve to reside in our brains because they are "right").

All that really exists are mental representations.The notion of meaning has come into picture only because of the innate desperation in human beings to communicate these mental representations to each other.Because when we want to communicate we must agree upon a way to codify these representations into abstract ideas that can be spoken out and written out in the language of our choice.There are many institutions which proclaim themselves to be the authorities on meaning.The oxford dictionary would be one.But,what they serve as ,is no more than an instrument to avoid confusion,an instrument meant to make an effort to smoothen out differences in meaning from culture to culture.
My response to the "Twin earth" thought experiment is this.Indeed for one it is,XYZ and the other it is H20,but what would unite the two ,is the fact that maybe both XYZ and H20,excite the same neural patern in the eight year old girl.
I agree with Jackendoff that internal concepts (I-concepts) are the only ones "worth a fruitful scientific enquiry.