I just heard that, in the case of Hilary Putnam's "Twin Earth" experiment, Tyler Burge argued that Oscar and Twin Oscar had different concepts in mind when talking about "water". This seems bizarre, doubly so if neither Oscar nor Twin Oscar are familiar with the chemical composition of the stuff they call "water". If they don't know the chemical composition of the stuff, and the chemical composition is the only different between the two substances (all mesoscopic properties are identical), how can their mental concepts of the stuff possibly be different?

Suppose we individuate concepts by "reference," so two mental states/thoughts are identical if they are about the same things, otherwise different. If (arguably) one twin's thoughts 'refer to' H2O and the other's 'refer to' XYZ, then they would count as different thoughts or concepts. What you are merely presuming is that the notion of 'concept' should be narrowly individuated (ie defined only in terms of what's 'in the head', so the two twins shoudl have the same concept). But that is the very thing that is explicitly being debated in the classic papers by Putnam, Burge, and all the rest ....!

best, ap

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