  The only general argument that seem to be available for this would be the principle that, for any two levels of discourse, A and B, they are more-than-causally connected only if one entails the other a priori. And the argument for accepting this principle would be that the relatively uncontroversial cases of a posteriori necessary connections are in fact cases in which one can argue a priori from facts about the microstructure to the manifest facts. In the case of water, for example, it would be claimed that it follows a priori that if there were something with the properties attributed to H2O by chemistry on a micro level, then that thing would possess waterish properties on a macro level. What is established a posteriori is that it is in fact H2O that underlies and explains the waterish properties round here, not something else: the sufficiency of the base – were it to  obtain – to explain the phenomena, can be deduced a priori from the supposed nature of the base. This is, in effect, the argument that Chalmers uses to defend the zombie hypothesis. The suggestion is that the whole category of a posteriori more-than-causally necessary connections (often identified as a separate category of metaphysical necessity) comes to no more than this. If we accept that this is the correct account of a posteriori necessities, and also deny the analytically reductionist theories that would be necessary for a priori connections between mind and body, as conceived, for example, by the behaviourist or the functionalist, does it follow that we can tell a priori that consciousness is not more-than-causally dependent on the body?