Aleph basically has it right in his reply: ‘water’ is a special case because it’s a rigid designator, picking out the actual watery stuff in all counterfactual worlds (even when some other stuff, XYZ, is the watery stuff instead of our water).
Conceiving of the “twin earth” world (where the watery stuff isn’t H2O) is indeed informative, since if this really is a coherent scenario then there really is a metaphysically possible world where the watery stuff isn’t H2O. It happens that we shouldn’t call that stuff “water”, if it differs from the watery stuff in our world, but that’s mere semantics. The reality is that there is a possible world corresponding to the one we’re (coherently) conceiving of.
Aleph basically has it right in his reply: ‘water’ is a special case because it’s a rigid designator, picking out the actual watery stuff in all counterfactual worlds (even when some other stuff, XYZ, is the watery stuff instead of our water).
Conceiving of the “twin earth” world (where the watery stuff isn’t H2O) is indeed informative, since if this really is a coherent scenario then there really is a metaphysically possible world where the watery stuff isn’t H2O. It happens that we shouldn’t call that stuff “water”, if it differs from the watery stuff in our world, but that’s mere semantics. The reality is that there is a possible world corresponding to the one we’re (coherently) conceiving of.
For more detail, see Misusing Kripke; Misdescribing Worlds, or my undergrad thesis on Modal Rationalism