 If two-dimensionalism is workable it solves some very important problems in the philosophy of language.Saul Kripke has argued that "Water is H2O" is an example of a necessary truth which is true "a posteriori", since we had to discover that water was H2O, but given that it is true (which it is) it cannot be false.It would be absurd to claim that something that is water is not H2O, for these are known to be "identical".