Materialists claim that world2 is reducible to world1. Work in Artificial Intelligence suggests that world2 is reducible to world3, and that one of the main explanatory roles Popper attributes to world2, namely causal mediation between worlds 1 and 3, is a redundant role. The central claim can be summed up as: "Any intelligent ghost must contain a computational machine." Computation is a world3 process. Moreover, much of AI (like linguistics) is clearly both science and not empirically refutable, so Popper's demarcation criterion needs to be replaced by a criterion which requires scientific theories to have clear and definite consequences concerning what is possible, rather than about what will happen.
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