 A number of philosophers argue that Kripke’s ideas about a posteriori necessary truth facilitate the defense of physicalism. They urge that even if a zombie world is conceivable, that does not establish that it is possible in the way that matters. Conceivability is an epistemic notion, they say, while possibility is a metaphysical one: ‘It is false that if one can in principle conceive that P, then it is logically possible that P; … Given psychophysical identities, it is an ‘a posteriori’ fact that any physical duplicate of our world is exactly like ours in respect of positive facts about sensory states’ (Hill and McLaughlin 1999, 446. See also Hill 1997; Loar 1990/1997; 1999; McLaughlin 2005; Webster 2006). Some philosophers reject even the assumption that conceivability is a guide to possibility, challenging the view that the burden of proof is on those who deny the zombie possibility (Block and Stalnaker 1999; Hill and McLaughlin 1999; Yablo 1993).