 One metaphysical question that is more directly relevant for the question of the phylogenetic distribution and evolution of consciousness is whether possessing it (i.e. being conscious) is binary (i.e. on/off, all-or-nothing), or admits of degrees.  Several authors have, for quite different reasons, denied what they take to be a common but problematic assumption — that “Consciousness is an on/off switch; a system is either conscious or not,” as Searle — who endorses the thesis puts it (quoted by Lycan 1996, who denies the thesis).