  Mention should here be made of influential criticisms of the identity theory by Saul Kripke and David Chalmers respectively. It will not be possible to discuss them in great detail, partly because of the fact that Kripke's remarks rely on views about modality, possible worlds semantics, and essentialism which some philosophers would want to contest, and because Chalmers' long and rich book would deserve a lengthy answer. Kripke (1980) calls an expression a rigid designator if it refers to the same object in every possible world. Or in counterpart theory it would have an exactly similar counterpart in every possible world. It seems to me that what we count as counterparts is highly contextual. Take the example ‘water is H2O’. In another world, or in a twin earth in our world as Putnam imagines (1975), the stuff found in rivers, lakes, the sea would not be H2O but XYZ and so would not be water. This is certainly giving preference to real chemistry over folk chemistry, and so far I applaud this. There are therefore contexts in which we say that on twin earth or the envisaged possible world the stuff found in rivers would not be water. Nevertheless there are contexts in which we could envisage a possible world (write a science fiction novel) in which being found in rivers and lakes and the sea, assuaging thirst and sustaining life was more important than the chemical composition and so XYZ would be the counterpart of H2O. 