2O. Again, he admits that the identification of heat with molecular motion and water with H
2O were both contingent a posteriori discoveries. However, this has no effect on their necessary identities. Indeed there is another contingent fact about these necessary identities. Kripke says that of course we can "imagine heat without molecular motion" and a mental state "without any corresponding brain state". But none of this affects the necessary identities. (Note: Kripke believes that the identity of heat with molecular motion is necessary but he doesn't think the same about the identity of a particular mental state with a particular brain state.)
2O and water. There may be other examples of stuff that has the macro-qualities of water, but it would not thereby be water. In Kripke’s case, the macro-properties of water are not the standard by which we determine or define water. That standard falls within the ambit of water’s micro-properties – that is, H
2O molecules. It is these micro-properties that make water a natural kind, not water’s macro—properties, which may, after all, be shared by other substances. Water is also H
2O whether or not we discover this to be the case. But why doesn’t all this apply to mental states and brain states? Because mental states are defined exclusively in terms of its phenomenal qualities, unlike water. That is, if we come across phenomenal qualities that don’t coincide with particular brain states, then such mental qualities are not necessary identical to such brain states. There is, however, a contingent identity between mental states and brain states. There is, therefore, no distinction between macro- and micro-properties when it comes to mental states.
P ⊃ P