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Full Idea
Final definition of 'Materialism': Among worlds where no natural properties alien to our world are instantiated, no two differ without differing physically; and two such worlds that are exactly alike physically are duplicates.
Gist of Idea
Materialism is (roughly) that two worlds cannot differ without differing physically
Source
David Lewis (New work for a theory of universals [1983], 'Min Mat')
Book Ref
'Properties', ed/tr. Mellor,D.H. /Oliver,A [OUP 1997], p.212
A Reaction
This would presumably allow for an anomalous monist/property dualist view of mind, but not full dualism. But if there are no psychophysical laws, what stops the mental changing while the physical remains the same?