 In a related challenge, Joseph Levine (1983, 1993, 2001) argues that, even if the conceivability of zombies doesn't entail that functionalism (or more broadly, physicalism) is false, it opens an “explanatory gap” not encountered in other cases of inter-theoretical reduction, since the qualitative character of an experience cannot be deduced from any physical or functional description of it. Such attempts thus pose, at very least, a unique epistemological problem for functionalist (or physicalist) reductions of qualitative states.