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?