Full Idea
For Chalmers, that water is XYZ is 'primary possible' (a priori, or conceptually), because it is true in some world considered as actual. It is 'secondary impossible', when it is evaluated from the Earth as actual.
Gist of Idea
Evaluate primary possibility from some world, and secondary possibility from this world
Source
report of David J.Chalmers (Does Conceivability Entail Possibility? [2002]) by Anand Vaidya - Understanding and Essence Intro
Book Reference
-: 'Philosophia' [-], p.813
A Reaction
[compressed] This is Chalmers' account of how we can know possibility from conceivability, via his two-dimensional semantics (see alphabetical themes).