  Critics of functionalism often deny that consciousness can be adequately explicated in functional terms (Block 1980a, 1980b, Levine 1983, Chalmers 1996). According to such critics, consciousness may have interesting functional characteristics but its nature is not essentially functional. Such claims are sometimes supported by appeal to the supposed possibility of absent or inverted qualia, i.e., the possibility of beings who are functionally equivalent to normal humans but who have reversed qualia or none at all. The status of such possibilities is controversial (Shoemaker 1981, Dennett 1990, Carruthers 2000), but if accepted they would seem to pose a problem for the functionalist. (See the entry on  qualia.)