In the addenda to his Naming and Necessity, Kripke provides an account of how necessary aposteriori statements are possible. In such a case, there is an apriori general principle telling us that it is necessary if true at all. Though straightforward in its broad compass, this account faces two obvious questions in its application: in each case of necessary aposteriori statements, what is the underlying principle and how is it established apriori? I treat these questions with respect to theoretical identity (...) statements concerning chemical substances, such as ‘water is H2OH2O’. I argue that the general principle underlying the necessity of the statements is that if a chemical substance has a certain chemical composition, then it could not have had any other chemical composition. Then I defend the view that the principle is a conceptual truth by providing a novel derivation of it from the theoretical concept of chemical substance with a sufficient level of formal rigor. The logical principles required for the derivation will also be stated and defended. (shrink)