Full Idea
Chalmers says that physicalism is true in a world w just in case every positive fact that obtains in w also obtains in any world physically indiscernible from w.
Gist of Idea
Physicalism says in any two physically indiscernible worlds the positive facts are the same
Source
report of David J.Chalmers (The Conscious Mind [1996], 2.1) by Karen Bennett - Supervenience
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.31
A Reaction
[Bennett summarises Chalmers' argument on pp.39-40] Chalmers says negative facts depend on the world's limits, which aren't part of the physical facts of the world.