 Of course, it is an empirical question whether names and natural kind terms are in fact governed by indirect reference-fixing rules of this sort. But according to Jackson, you can test whether your implicit understanding of ‘water’ is structured in this way by considering possible situations in two different roles: as your actual environment or as a mere counterfactual possibility (Jackson 1998a, ch. 2). Consider two different possible worlds based on Putnam’s Twin Earth thought experiment (Putnam 1972). In the first world, Earth, the clear potable stuff that fills lakes and streams and is habitually called ‘water’ by English speakers is H2O. The second world, Twin Earth, is exactly the same except that the stuff that has these properties is the complex chemical kind, XYZ. If your commonsense understanding of ‘water’ is governed by the proposed reference-fixing convention, it would lead you to identify different chemical substances as water depending on what your actual environment is like: if your actual environment is Earth, then water is H2O; but if your actual environment is Twin Earth, then water is XYZ. If you assume that water is actually H2O, moreover, you will judge that water is essentially H2O in all counterfactual circumstances. And if you assume water is actually XYZ, then you’ll judge water is essentially XYZ.