  Carruthers has championed the view that only humans (with the possible exception of chimpanzees) are conscious, although for different reasons than Dennett. Carruthers (1998a,b, 2000) has argued to this effect based on his ‘higher-order thought’ theory, according to which, phenomenal consciousness requires the capacity to think about, and therefore conceptualize, one's own   thoughts.[8]   Such conceptualization requires, according to Carruthers, a theory of mind. And, Carruthers maintains, there is little basis for thinking that any nonhuman animals have a theory of mind, with the possible exception of chimpanzees (see Lurz 2011 and Andrews 2012 for in depth discussion of theory of mind in nonhuman animals). This argument is, of course, no stronger than the higher-order thought account of consciousness upon which it is based.  But setting that aside for the sake of argument, this challenge by Carruthers deserves further attention as perhaps the most empirically-detailed case against animal consciousness to have been made in the philosophical literature.