Now the question arises: when an Earthling (or Oscar for simplicity's sake) and his twin on Twin Earth say 'water' do they mean the same thing? (The twin is also called 'Oscar' on his own planet, of course. Indeed, the inhabitants of that planet call their own planet 'Earth'. For convenience, we refer to this putative planet as 'Twin Earth', and extend this naming convention to the objects and people that inhabit it, in this case referring to Oscar's twin as Twin Oscar.) "Ex hypothesi", they are in identical psychological states, with the same thoughts, feelings, etc. Yet, at least according to Putnam, when Oscar says 'water', the term refers to H2O, whereas when Twin Oscar says 'water' it refers to XYZ. The result of this is that the contents of a person's brain are not sufficient to determine the reference of terms they use, as one must also examine the causal history that led to this individual acquiring the term. (Oscar, for instance, learned the word 'water' in a world filled with H2O, whereas Twin Oscar learned 'water' in a world filled with XYZ.)