  In spite of these similarities, the Exclusion Problem is in one important respect unique: unlike the problems we’ve looked at so far, exclusion worries generalize to a wide range of phenomena outside of the mental. Any properties, mental or otherwise, that are multiply realizable in physical systems are threatened with causal irrelevance. (For discussion of this and related issues, see Kim 1998, pp. 77–87; Noordhof 1999; Bontly 2001; Gillett and Rives 2001; Block 2003; Walter 2008.)