We begin by supposing that elsewhere in the universe there is a planet exactly like Earth in virtually all aspects, which we refer to as "Twin Earth". On Twin Earth, there is a Twin equivalent of every person and thing here on Earth. The one difference between the two planets is that there is no water on Twin Earth. In its place there is a liquid that is superficially identical, but is chemically different, being composed not of H2O, but rather of some more complicated formula which we abbreviate as "XYZ". The experience of people on Earth with water and that of those on Twin Earth with XYZ would be identical. Now the question arises: when an Earthling and his twin on Twin Earth say 'water' do they mean the same thing or not?

We begin by supposing that elsewhere in the universe there is a planet exactly like Earth in virtually all aspects, which we refer to as "Twin Earth". On Twin Earth, there is a Twin equivalent of every person and thing here on Earth. The one difference between the two planets is that there is no water on Twin Earth. In its place there is a liquid that is superficially identical, but is chemically different, being composed not of H2O, but rather of some more complicated formula which we abbreviate as "XYZ". The experience of people on Earth with water and that of those on Twin Earth with XYZ would be identical. Now the question arises: when an Earthling and his twin on Twin Earth say 'water' do they mean the same thing or not?